Engine misfire troublshooting

[Kudzupatch]

Originally Posted by ChevyTech

This is a sign that the engine has a mechanical knock and the system is pulling timing.....

Thanks. That makes sense and had crossed my mind but I had forgotten about it. I don't hear anything but I realize I might not hear it.

I just drove to town to pick up some lumber and the stumble is MUCH worse. Also seemed down on power which would to line up. I am going to bump it back up to 4 degrees for now to improve the driveability.

I will go ahead and order a cable and get TunerPro installed and go from there.

[ChevyTech]

When you get Tunerpro, look at knock counts.

If it will show what the system is doing to the timing that would be a big help too.

Some knock counts are normal. From time to time the system will add timing until it gets a knock count, and then back off the timing a little. With better tools you can clear the counts and watch how fast they are counted again.

Remember anything that rattles, on, or in, the engine, or even the transmission, can get picked up by the knock sensor and counted as a ping.

[Kudzupatch]

I GOOFED!

I went out to set the timing back to 4 degrees and I noticed a vacuum line was loose. Apparently when I was setting the timing yesterday I hit the line to the filter canister it pulled it loose. Quick test drive and it is running like it was. Still stumbles, but timing didn't make it worse after all.

Still going to order the cable because i am out of ideas as to the stumble.

[Kudzupatch]

WE HAVE DATA!!

TunerPro has been an experience but finally got to working and made a short trip and captured some data.

First things first. Knock counts seems crazy high to me but I don't know. Shows 680 after 3 miles. That is the point I started to record the data. But I don't know if that was 0 when I started or not? Regardless, it was on 1250 by the time we got back from a 15 mile trip to town. I have a strange click we are not sure what is, so that may be the reason for that.

Next thing, playing back the log I can plainly see at the stumble. The RPMS go up. My timing drops down to as low as 2 degrees and then pick ups as the stumble quits.

BUT, if I am reading this correctly, it is not registering any knocking during this time. As I play back the recording the knock count doesn't change. So what would be causing the timing to back up?

Next thing I notice is the 02 sensor reading area all over the map! Even at idle is going from 40mv to 800mv. Driving down the road it is all over the map. That isn't normal is it? I wouldn't be surprised to find that is is the original. Owner took EXCELLENT care of his car. But I don't think he ever had anything but plugs put in it. I don't know, but I think it is a case of, if it running good, leave it alone.

[Kudzupatch]

Spent some more time reading the data and talking with a buddy that knows more about these.

We get 280 knock count cranking the car! So there is obviously some false reading going on. Need to look for what is loose and making the noise.

Looking at the data we notice the o2 reading show very lean and staying there, when the stumble happens. It'snot jumping all over the place like it normally is. The o2 and timing change at almost the same time. As if the two are tied together. Engine goes lean and timing drops. It riches up and the timing comes back in and engine recovers.

No knock counts during the stumble either time I have it recorded.

Sounds like we need to put an O2 sensor in it. Worst case is we verify the readings we are getting now are accurate.

If it not a bad O2 sensor, what else could be making the timing drop so dramatically, or what would make the engine go so lean?

[ChevyTech]

If it really is getting excessive knock counts you need to find the source. A knock sensor wire routed to close to a plug wire can cause knock counts.

Knock counts can be set by something loose rattling on or in the engine or transmission. Lifters, cam, rod, crank and piston noises all have the potential for causing false knock signals to be produced.

Fix the lean problem. The system may be protecting the engine by removing timing advance to protect the engine from the lean condition.

As per comments on the oxygen sensor voltage in the earlier post:
The voltage should jump up and down.
If the voltage varies from .3 to .7 of a volt it is referred to as a lazy sensor. If you see readings swinging from close to .1 up to .9 of a volt, the sensor is ok. Note: some low quality scan tools do not sample as often as good scan tools, and the voltage may need to be monitored for a longer period of time before condemning an O2 sensor.

If the oxygen sensor is giving a false low voltage reading, the engine will actually go rich.
If the oxygen sensor shows it is lean, and it actually is going lean, then don't blame the oxygen sensor for the problem. Find out why it can't get the fuel to correct what it detects.

On page 2 of this thread you had a code 15 and 33 set. I am thinking you may have several marginal electrical connections. These codes could be cause by poor connections or stretched out wire terminals.
Another thought came to me that you could be having a momentary lose of connection at some connections at the distributor module (which is a common problem). A poor connection on the wiring to the injectors will do exactly what you are experiencing.

My gut feel right now is, the lean condition is what you should peruse.

[Kudzupatch]

Read this a couple of times and I follow your thinking.

I think the past codes were caused by me when I was messing around with things loose to see what would happen when I was hunting that misfire that was caused by the Dist. Cap. But no harm in checking and cleaning the connections. There hasn't been any codes since I have driven it a good bit.

I think some of the knock counts on cranking is caused by valve noise. If it has been sitting for long the lifter bleed down and for few seconds it is really noisy till the oil pressure comes up. But there is still a lot coming from something else.

[Kudzupatch]

The Saga Continues!

On a whim or moment of inspiration I decided to plug the EGR vacuum line and drive it a couple of days. Stumble is 90% gone. So this leads me to think I have the 'wrong' replacement EGR. Haven't hooked up the computer to see what is going on yet but the difference is obvious.

There is still something else going on and I am very suspicious of the ignition system. There is still that issue of where the high knock counts are coming from. So I still have some work to do but if feels good to finally see some progress.

[ChevyTech]

On page one you said you were going to replace the EGR and that it had a negative back pressure EGR system.

Did you replace the EGR valve?

Did you use a OEM/GM/AC Delco EGR valve if you did?

That system is very picky about EGR operation.

The knock counts should be higher with the EGR plugged.

If it has high knock counts, it will pull timing advance. Find out where the knock counts are coming from.

[Kudzupatch]

BAFFLING UPDATE

Been very busy and haven't had time to do much work on it. A few days ago I reconnected the EGR. Driving around with it plugged helped a lot but did not stop all the hesitation. BTW the replacement EGR was not a GM.

Since I was having a lean condition and the computer was dropping the timing dramatically from a standing start. I decided to set the timing back to 4 degrees, where it was when I started all this. Figuring it would make driveablity better till I could get a new GM EGR.

To my surprise, no hesitation and I no longer feel the random loss of power on start up. I have driven if for a week now and no issues. The mad rush of work has slowed so I want to connect the laptop and data log a drive to see what is going on. I want to know if the advanced timing is masking the issue. Also makes me wonder if the timing mark could be off? I can't see the harmonic balancer but I know the old ones had an outer ring that could slip and throw the timing marks off.

[ChevyTech]

Originally Posted by Kudzupatch

I want to know if the advanced timing is masking the issue.

Yes.
What you discribe is common on engines that have a mechanical knock that is causing knock counts.

Originally Posted by Kudzupatch

Also makes me wonder if the timing mark could be off? I can't see the harmonic balancer but I know the old ones had an outer ring that could slip and throw the timing marks off.

Yes the out ring can slip on these trucks.

[speedygonzales]

Months ago

Originally Posted by Kudzupatch

Double checked the EGR this morning by pluging the line and no change.

Recently

Originally Posted by Kudzupatch

The Saga Continues!

On a whim or moment of inspiration I decided to plug the EGR vacuum line and drive it a couple of days. Stumble is 90% gone. So this leads me to think I have the 'wrong' replacement EGR.

I think it's funny in a twisted sort of way that it was suggested to look at things months ago that you eventually get around to and it's like wow. EGR and O2s come to mind.

[Kudzupatch]

Originally Posted by ChevyTech

Yes.
What you discribe is common on engines that have a mechanical knock that is causing knock counts.


Yes the out ring can slip on these trucks.

OK next step is pull the distributor since that 'seems' to where the noise is coming from.

A friend said there is a shaft between the oil pump and the distributor and mentioned something about plastic that breaks down at the joint and that is what he suspected was making the noise. My experience is in early small blocks and I remember the dist. mating up with the oil pump shaft and nothing between them?

[mountie]

I read the whole post.....

I have a similar situation.....the stumble / miss...... similar things replaced...new..... or checked....

I have my 1988 GMC 2500 SLE Sierra at a shop.....

I'll let you know what was found when ( if) the miss is gone.

[Kudzupatch]

Thought I would update this even though I have really made no progress.

Started to have a very noticeable misfire at idle which was new. After so investigation I went ahead and replaced the distributor. It stopped the misfire with no effect on hesitation. But that is sort of what I expected.

Plugged ERG help at first and then after a few days started the hesitation again. Even put the old one back on to see if there was any difference and none. Yes, that one has a slow leak when tested but I figured I had nothing to loose.

Still have the high knock count and can't find what might be causing it.

Had to top off the freon and found the compressor was acting up and starting to slip or rather freeze up. So I am putting a new on one. Also found an idler with a noisy bearing. Unless one of those turns out to be the cause of the knock I am really stumped.

[ChevyTech]

Finding the cause of knock counts can be one of the hardest diagnosis you will ever do.

I have been the consultant on engine tear downs looking for the cause of knock counts.

Anything that rattles or it loose has the potential of causing high knock counts. It does not have to be in the engine.

Inducted voltage can cause knock counts.

[Kudzupatch]

Originally Posted by ChevyTech

Inducted voltage can cause knock counts.

Good grief! Where is the rolling eyes emoticon? That does make me feel a little better that I can't find it though.

Anything that rattles or it loose has the potential of causing high knock counts. It does not have to be in the engine.

My shop has a big wooden boat project taking up most of my floor space. But I want to bring it in and put it up on stands and go over everything underneath.

I do have one possibility that concerns me. When I crank the engine cold there is 'knock' for lack of a better word till the oil pressure cranks up. Wondering if there is a loose rod bearing in the bottom end. It not a low mileage engine at 175K but my last one went over 300K but it was slap worn out. This one appears to be excellent shape other than this annoying issue.

But I have looking at crate engines just in case.

[ChevyTech]

Below is a clip from an article I showed to a group of guys at a dealer that had me consulting on a engine tear down looking for knock counts and a code 43 they could not get rid of. Two Chevrolet dealers had already given up on this truck, before the third dealer that called on me, took it on.

The answer to the knock counts that two dealer gave up on:
The baffle in the oil pan had came lose on one side, which was the cause of the knock counts. It would rattle on medium to hard acceleration.



Chevy Pickups With The "Mysterious Code 43"
It’s not uncommon for Chevy pickups with injected 350s to set a Code 43 indicating erratic spark or a detonation problem after the installation of a re manufactured engine. The installer usually blames it on the engine because it has "the wrong heads" or "the wrong compression ratio" after everything else seemingly checks out okay.

Rebuilder’s survival tip: The main wiring harness runs along the right valve cover. It has a "dip" in it that is supposed to go under the heater hose. If the installer turns the harness upside down so the "dip" goes over the heater hose, the harness is close to the plug wires and it will induce a small current into the harness when the engine is run hard.
This small voltage in the circuit for the knock sensor is all it takes for the computer to "think" that there’s erratic spark or detonation present, even though there really isn’t a problem, so it sets a "Code 43" after a road test. The computer always checks the spark control circuit at start-up and sets a Code 43 if there’s a problem in the electronic spark control circuit, but it should never set a Code 43 after it’s been running down the road. If it does, then you’ve got the "Mysterious Code 43" that is probably caused by the high voltage plug wires inducing a current into the harness. We have fixed several of these pickups with this problem by rerouting the harness under the heater hose.

[Kudzupatch]

I hate when people don't update a thread but I am starting to realize a lot never cure their problem. I assume most people have been like me and just living with it.

No luck on the high knock count. I did look at the wiring harness but I doesn't seem to apply to my Suburban. Harness doesn't seem to be in the same place. But I did notice the spark plug wires were closer to the harness because the factory wire looms have fell apart from age and heat. So I need to reroute them just in case that is a problem.

I bought a new Delco EGR this time and that cured the off idle stumble. It is so nice to drive a car that goes when you step on the gas! Other than tune-up parts I have just replaced the distributor and the EGR. While it didn't fix the problem, the distributor did make a noticeable difference and I think cleared up an underlying problem.

I have a new issue that has just started a few weeks ago. The rough idle I read so much about. Will sometime buck and go dead at a red light. It idles smoothly and then suddenly bucks and kicks and then idles OK. IAC seems to be working properly. No other apparent issues either.

I thought I would try cleaning/rebuilding/resealing the TB. I have read about carbon build up causing this issue or sometime there is a leak I have not found. Cheap repair and it will eliminate that if it isn't the problem.

Hope this helps someone.

[mountie]

Backfire through the intake..... starving for fuel. 'lean'

Backfire through exhaust.... too much fuel ( fuel not burnt ) 'rich'