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.