Engine misfire troublshooting
 

93 Suburban, 170K on very well maintained vehicle. A few weeks back I gave it full tune up, wires, cap, rotor, plugs etc. I took it on trip into the mountains and it started misfiring under light to moderate loads. For example when we hit a bit of a climb and had to step down on the throttle a little bit.

Stop and when it cools down it runs fine till it gets warm again. No indications it is overheating at all. But the warmer it was the worse the problem seemed to be. Sounded like the ignition module so I replaced that and it seemed to fix it.

Then, everyone once in a while I would get a backfire through the TBI. I finally started seeing a pattern. While it only does it occasionally it always happens under acceleration or an increase in throttle and load and only at lower throttle openings. If you step down hard on it doesn't do it.

This week I made a 300 mile road trip on the interstate and it starts doing it again but consistently after 200+ miles. Rough idle, rough running at low to moderate loads with occasional backfires when you increase the load or start up from a red light. I did have a small load in the vehicles and 300 lb trailer behind it. Again, more throttle and it smooths out.

Got another trip coming up in a couple of weeks so I am anxious to find the problem. Fuel pressure gauge is on order. Will start there and report what I find. What else could cause this?

Re: Engine misfire troublshooting
 

did you change the fuel filters ? possible fuel pump issue , check the fuel pressure and report back

Re: Engine misfire troublshooting
 

Sounds like an ignition module, aftermarket are crap.

Re: Engine misfire troublshooting
 

No, and that is first on my list. But I am going to install the pressure gage first and see what I have. That way I will know if the filter changed anything.

I have considered the module. I didn't know about the difference in GM and aftermarket modules before I swapped it out. BUT, it was doing this with the GM module too.

Got an hours drive this morning with the load and trailer and very anxious to see if it acts up. Then tomorrow I make the 300 mile trip back.

Re: Engine misfire troublshooting
 

Waiting on the Pressure gage to get here and wanted to check a few sensors and double check vacuum connections.

I found a cracked hose fitting on the PCV line. There is at least a small vacuum leak so that could at least part of the problem. It does seem worse at part throttle/high vacuums. That will be replaced today.

Also found the temperature sensor appears to be off? Water temp in the radiator is 65 degrees. The sensor reads 1972 ohms which, if I am reading the right chart is 95+ degrees. 30 degrees off, but is that enough to warrant replacing it?

I will let it warm up and recheck after I do a few other things.

Running 370 ohms @ 160+/- degrees and 294 ohms @ 182 degrees. It topped at at that point. 190 thermostat. This seems more in line with what the chart.
Checked at the radiator with a temp probe in the water.

TPS - I didn't have a good way to test voltage but the I watched the ohms move steady as I opened the throttle and closed it.

MAP was not checked. Don't have a good way to test it without piercing the wires.

Do see some dampness around the pressure regulator so no doubt the gasket is leaking.

As I said. Waiting on the fuel pressure gage.

Re: Engine misfire troublshooting
 

Don't overlook the possibility that one of the new spark plugs is bad ,or has a cracked insulator.

There are a lot of spark plug sockets that are not deep enough which makes it easier to crack a spark plug when installing them.

Re: Engine misfire troublshooting
 

Good point and one I would have probably not thought of.

I decided to go ahead and put a new fuel filter on it. Not sure but I suspect it was the factory unit. No markings on it except for an ink stamped part(?) number. It wasn't totally stopped up but it was definitely a source of restriction.

Replaced the PCV hose too and there is now a noticeable difference in the way it runs. Throttle response seems .....crisper. Hard to define but I noticed it just as soon as I pulled out of the driveway. Just seems to respond better.

I should have the pressure gage Monday and about to make the same trip I made last week. This time no trailer but 7 hours of driving should reveal if the problem is fixed or not. By the time I got there last week it was not running smooth at idle or low speeds. Long drives seems to make it worse.

Thanks for the input!

Re: Engine misfire troublshooting
 

OK, still looking. Started it again on the way back from Church. Decided to bypass the EGR valve and we can mark that off the list. I unplugged the controls and I tried it with them connected and the vacuum line disconnected and plugged.

Good thing is it seems to be getting worse which makes it easier to trouble shoot.

Fuel pressure check Tuesday if all goes as planned.

Re: Engine misfire troublshooting
 

I have been thinking more about the EGR valve today. Tomorrow is truck day and I have all day to work on it. I have the pressure gage in hand so tomorrow I am going to install that and if see where pressure is. That may give me my answer. If that is not the issue then I am going to pull the EGR and inspect it.

If nothing there, I am going to to scrutinize the distributor and look for a cracked plug.

The list is getting shorter and shorter.

Re: Engine misfire troublshooting
 

really sounds to me like egr too i had a 93 jimmy that did the same it had the 3.4lt and they were famous for egr issues but from what your saying i think too it is the egr

Re: Engine misfire troublshooting
 

Still working but taking a break. 11-12 PSI while running. Did not let it run long because I didn't want it to get hot yet.

I took the EGR off, connected it to a powered vacuum pump (wife food saver) and it pull up but will not hold once the pump shuts off. I verified that the pump does hold the vacuum once the motor stops. So the EGR is leaking and will get one for it.

Hopefully that is the problem.

I am going to pull the cap and check the rotor look for play in the shaft. I have done this before but I want to check everything that could be causing this.

Another suggestions are welcome while I am at it.

Re: Engine misfire troublshooting
 

Waiting in an EGR valve. Checked ignition and nothing obvious there. No play in distributor shaft. Cap and rotor just show normal wear.

One thing I did see and I don't know if this is normal or not. Around the dist. pickup there is a metal ring, you can see it on the outside of the pickup. Just above the ignition module in the photo I "borrowed".

Is it supposed to be able to move? I have searched and searched and can not find a breakdown of what is in there. It reminds me of the old centrifugal advance but there is no spring return. I can rotate it 15 degrees? It slides and hits something solid and stops. Rotates rather freely and that seems odd but it may be a dust shield and not hurting anything.

http://easyautodiagnostics.com/gm_ic...es/image_9.jpg

Re: Engine misfire troublshooting
 

Before you mess with the EGR check ChevyTech's post here: http://67-72chevytrucks.com/vboard/s...&highlight=EGR

Re: Engine misfire troublshooting
 

I have read most everything there is on trouble shooting on this board. Especially anything he posted.

Re: Engine misfire troublshooting
 

Negative back pressure EGR valves should hold vacuum when the engine is off. Have you checked which EGR system it has?

Most 1/2 ton trucks have a negative backpressure EGR system. If your truck is a 2500, you need to be careful, as it could have a Port EGR system.

If your EGR valve is leaking vacuum, it should have less operation which could cause a code 32 or pinging. This system will cut timing advance as soon as the knock sensor detects pinging. This would cause a lack of performance but should not feel like a misfire.

It is normal for the outer shield on the distributor to move like you stated.

Sorry I have not been getting back to this thread quickly. I have been very busy.

If you want some long threads that cover a lot check these:
http://67-72chevytrucks.com/vboard/s...d.php?t=347247
http://67-72chevytrucks.com/vboard/s...d.php?t=348308

Re: Engine misfire troublshooting
 

If you want some ideas.

My first guess would be plugs or wires. Like I said earlier, it could be a cracked plug. I have had to diagnose cracked plugs that happened during tune-ups by fellow techs. A cracked plug can cause a miss on light acceleration just after it gets into high gear and lock-up, and not show up under other conditions.

Any new part like the distributor cap should be suspected too.

Double and triple check the firing order.

Always separate #5 and #7 plug wires and cross them rather then let them run parallel for a long distance.

When the head gasket start to fail between the two center cylinders, a bit of a misfire feeling can be felt and sometimes it also causes pinging. You may want to test the compression is everything else checks out good.

Re: Engine misfire troublshooting
 

Thank you for all the input! I have read a lot of what you have written and GREATLY appreciate it. I love technical writing so I have dug into the Sticky's.

It is a negative system and on the way to pick up the EGR it started showing it first codes. 32 and 33, EGR and MAP sensor. I am about to put the EGR on and will double check my MAP connections since I had it loose. Might not have a good connection.

Wife has to drive it the next couple of days while hers gets a new power steering pump. Once I get it back if this doesn't take care of it, I will check the plugs and compression.

Chevy Tech, don't know where you are but I owe you a case of your favorite beverage or a steak dinner for all the info you have shared!!

Re: Engine misfire troublshooting
 

No love in Mudville tonight. Still sputters and backfires just the same. But the EGR needed to be replaced so it wasn't just throwing money at the problem.

BUT, I had an idea thinking about the possible ignition issue. We went to supper and when we got back I pulled around to shop, misted the ignition wires with water and had my wife power brake it (in reverse) raising the throttle till it started to miss.

To my surprise, once my eyes adjusted to the dark, I saw faint sparks from the wires in several places. Every once in a while I could hear a spark from around a plug on the drivers side. I saw it once but not sure which cylinder. I saw sparks around the coil too. Arcing to the transmission filler tube, etc. These are only a year old and supposed to be good wires.

Wife needs it while her Extera gets a Power Steering pump transplant. I am thinking about ordering a set of good wires. And since I need to take out and check each plug I think I will just put new plugs in.

Re: Engine misfire troublshooting
 

I just posted today in Ripsaw's thread about water testing the plug wires, and looking at them in the dark. Maybe you saw that already. Small sparks dancing around is normal but the spark you heard by a spark plug sounds like a problem.

Make sure the heat shields are not bent making them closer to the spark plugs.

Good wires should come with some dielectric grease to put in the boots.

My Reply to Ripsaw:
http://67-72chevytrucks.com/vboard/s...2&postcount=16
In this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=626302

Re: Engine misfire troublshooting
 

I remembered using water from years ago in my hot rod days.

At least I think I am on the right trail. Maybe Friday I can track down the issue, I am thinking there is a good chance it is cracked plug as you suggested.

Again, thanks for all help.

Re: Engine misfire troublshooting
 

New problems after maintenance are nearly always what you just "fixed". It's practically the law.

Jeff

Re: Engine misfire troublshooting
 

on the egr there is a metal tube did you check to make sure that was clear when you changed the egr valve just a thought

Re: Engine misfire troublshooting
 

I had a similar issue with my 87 tbi about a month ago. After warming up, wouldn't idle. Felt like it would die at a stop light. Found leaking injector o rings. At start up, would do ok, but ran like crap when it went into closed loop because it could not compensate for the extra fuel. Checked regulator gaskets and replaced injector seals. Runs great.

Re: Engine misfire troublshooting
 

Pointed that out to several people. Especially firing order issues after putting on headers or new wires.

But in this case, it been quite a while since it was tuned up. It has run fine for several months. This problem is fairly recent and started out very intermittent but has become almost constant. So what ever it is is getting worse so it will be easier to find.

Our other vehicle should be fixed today so I will have it back tomorrow. Will pull the plugs and check the wires. I really think that is going to be where the issue is.

Re: Engine misfire troublshooting
 

I checked all the wires and they look perfect, not skins or breaks. Checked the resistance and they are all very consistent. Long leads slightly higher as expected.

No cracks in the plugs, all the electrodes look the good. #8 however was a little wet and smelled of gas. Color was good on the plug though. I am thinking I need to test compression.

I did find the cap had an extremely high resistance reading on the coil tower. Something like 4 million ohms. So I installed a new cap and rotor and it runs better but still have the off idle stumble and backfired once on me during the test drive.

I have run out of things I can check today. About to drive it and run some errands and see what happens. Leaving on a trip Wednesday and was really hoping to find the problems this morning.

Re: Engine misfire troublshooting
 

I guess I misunderstood your original post where you said you gave it a complete tuneup "a few weeks back". I was under the impression that you made changes, then began having problems during a trip.

Re: Engine misfire troublshooting
 

Me too!

Re: Engine misfire troublshooting
 

Sorry about the confussion.
Tuneup was several months ago so this is a new issue.

UDATE:
Replaced ERG valve earlier this week with no change.

Today I replaced cap and rotor and as stated in previous post, checked the wires, plugs and everything I could see in the ignitions system. Now t is running much better but not quite perfect.

It backfired though the TBI one time within 2-3 minutes of starting the test drive and didn't seem much improved. I took a second drive and have driven it several miles through town and on the highway. It has not backfired again and no more rough idle either. Could it be that the computer had to adjust it's self after the repairs?

Still one issue left. It 'stumbles' or cuts out from a standing start with normal light throttle start. Not always but most time. It only does it for a couple of seconds and then it runs fine till the next stop and start. Similar to a carburetor when the accelerator pump quits. I did notice if you are moving even 4 or 5 mph it doesn't do it. Seems to be only from a dead stop.

Much better than it was though!

Re: Engine misfire troublshooting
 

Spoke too soon. Drove it some more and stumbling or as my wife calls it, 'Stuttering' kept getting worse and it about where it was.

Now I am getting two codes 15 and 33

Engine coolant temperature (ECT) sensor -voltage high
Manifold absolute pressure (MAP) sensor -voltage high

As soon as it cools enough to remove the cap I will double check the Coolant sensor.

Decided to try it with the O2 sensor disconnected and it ran perfect. Stopped at a buddies near me and we were discussing it and looking and I saw I had left the ECT disconnected.

Hook it back up and it runs bad. Hook up the O2 also no change. Take the ECT loose and it took the computer a few seconds to adjust, then it runs fine. Have I found the problem??

When I checked the resistance it seemed to be very close to right.

Also, been thinking about the distributor play. How much is a problem? Hard to tell if there is any movement. Was thinking about putting a dial indicator on it in the AM to see exactly how much it moves. Of course it maybe (hopefully) a mute point.

Re: Engine misfire troublshooting
 

Checked MAP sensor and seems to be working fine.

Checked the ECT again with with 2 thermometers)
125 degrees I am showing 404 ohms.

Per the chart 122 degrees should be 973 ohms.

404 ohms should be between 176 deg. and 158 deg. F.
Chart says 332 ohms @ 176 deg. F and 467 ohms @ 158 deg. F

Re: Engine misfire troublshooting
 

If you ran the engine with the coolant sensor unplugged, that will set the code 15.
If it sets a code 15 without the sensor being unplugged, then there is a problem that needs to be fixed.

Testing the coolant temp with a thermometer in the radiator is not a valid test. The coolant in the engine can be much warmer then the coolant in the radiator.

If you can't feel any sideways movement in the distributor, don't worry about measuring it.
If it wiggles much it should be replaced.

You better test the vacuum supply to the MAP sensor.

Re: Engine misfire troublshooting
 

Yea, I should have thought of that. I could see how there could be a 30 degree difference.

So, help me understand why the engine runs so good with the sensor unplugged? Am I wrong in thinking that it means that one of the sensors is sending a faulty signal to the computer? With the sensor unplugged it runs great. Plug it back up and all the trouble starts again.

I am really starting to relate to all the frustration others are feeling troubleshooting these engines.

Re: Engine misfire troublshooting
 

The system has detected a problem.

The system no longer trusts the sensor data and system runs at a predetermined calibration. This calibration is what will keep the system running best across the most normal conditions, without sensor data.

Yes, you are correct that the ECM is most likely getting bad data from at least one sensor.

When the sensor tests to be good and still seems to be causing a problems consider it may be a bad connection to the sensor or a bad ground or grounds. Test the ground side of the sensor too.

Here is how a oxygen sensor can or misfire can affect the system:
These systems work great when every thing is right but they have a big flaw. Once the engine misfires for any reason it throws the system in the rich direction. This system watches the O2 sensor, but the O2 only will see how much oxygen is in the exhaust stream NOT burnt gas. When every thing is right the O2 knows just how much of the oxygen should have been burned by combustion. If the system gets a little to rich all oxygen is consumed. If it gets to lean there is not enough fuel to burn all the oxygen and there is more oxygen in the exhaust. Once it misfires, there is way more oxygen in the exhaust, because none got burned in a cylinder. The system thinks it is to lean so it gives the engine a richer mixture. The richer mixture often makes it misfire more (especially if it is cause by weak ignition, bad plugs…) so it dumps in more fuel. Now it is so rich the plugs and O2 sensor get soot on them. It misses more. The system goes richer and richer.

If you don't seem to be getting a handle on what is going on, you should follow where the codes bring you. Clear the codes, run the truck, and test for codes again to see if they get reset.

The TBI system is the easiest fuel injection system to work on with the exception they did not put a fuel pressure test port on it.

Re: Engine misfire troublshooting
 

Great. That makes sense.

I will go check that now. I know grounds can create lot of issues and I always forget to check that.

That makes a lot of sense! Very good to know!

Will do! Going to go ahead and put in the temp. gage sensor and I will hook up everything and see what happens. Tempted to put in a new ECT while it is drained but I am really starting to doubt that was the issue now.

Once again, thank you very much! Always wanted to know more about how these work but.... I would have prefered to learn it from a book, not first hand. :lol:

Re: Engine misfire troublshooting
 

Saw something that gave me an idea. I took a piece of copper welding wire and it used it to probe the connections, just slide it down beside the wires till it hit the terminal. So, I have rechecked my sensors, checked for ground back to the negative post on the battery and they are good.

Borrowed a timing light since mine is MIA for some reason. Timing is set at about 4 degrees. Can't get to the nut to adjust it. I am sure Jim has a wrench I can borrow.

Reset the computer before checking the timing. With it connection back on it stays around 4 degrees at idle. With throttle it advances and is stable, not jumping around.

I climbed in the engine compartment, removed the cap and rotor and there is a small amount of play in the shaft. Very small, but from what you are saying any play is cause to replace it?

I don't mind spending the money but I just don't want to throw money at it and replace good parts. So I held off on the ECT sensor for now.

Taking it for a drive shortly and will check codes. But I am really wondering if it is the ignition/distributor now.

Re: Engine misfire troublshooting
 

Why don't you look for play in the timing chain rather than just the distributor?

Re: Engine misfire troublshooting
 

If I understand correctly, a bad bushing in the distributor can cause the electrical components to touch and lead to ignition issues.

I am pretty sure there is at least some slack in the timing chain. I don't see it causing this issue.

Re: Engine misfire troublshooting
 

Quick update. The rotor cap was obviously a big part of my issues. As I stated out of desperation I checked the resistance on each of the terminals on the rotor and the coil tower had over 4 million ohms resistance (EXTREMELY high). That has taken care of 95% of the problems. No more Check Engine light either.

There is the stumbling on starting up and the more I drive it, I think your right ChevyTech and it will be a cracked plug. It feels like a skip when it does it. I have pulled them but can not find a flaw. So I have a new set to put in and then I have a long trip ahead of me. So that will tell if it is fixed.

Re: Engine misfire troublshooting
 

Thanks for the up-dates.
I am keeping an eye on this thread.

Re: Engine misfire troublshooting
 

No love for the home team. Put new plugs in this morning before leaving and it stumbled pulling it around front.

Drove it 350+ miles with a load and trailer, just like I did 3 weeks ago. MUCH improved though. Ran perfect out on the road. Just have that stumble when I start up and when we came off the interstate into some stop and go traffic I noticed it was idling a little rough too.

FWIW it was pouring rain most all the way. Didn't seem to make it any worse though.

When you step on the gas is stumbles most times. Sometimes really bad and other just barely. Stepping down on the gas will make it go. So it is only happening at very first of the throttle opening.

Time for a compression check. I found my vacuum gage so will put that on it when I get home next week.

Could it be an O2 sensor? Buddy of mine is really leaning toward that but I am not sure if that could cause it?