1996 K1500 - 4.3L engine is missing - any thoughts?

[FriendshipUSA]

Engine missing problem with K1500 4.3L. It may be one cylinder. The engine runs ok some times then it starts missing. The miss is most noticeable at an idle. Engine light Code shows P0305. I replaced almost all the electrical components including plugs, wires, distributor cap, rotor, the coil and coil module, and computer. The distributor was not replaced. I don't know where the grounding points so these were not checked. I performed wire wiggle test to the computer, coil, and coil module cables. No change.

Does anyone have any ideas what might be the cause?

Scan test results:
Code: P0305
Freeze Frame data:
STFT Bank 1: -1.5%
LTFT Band 2: -1.5%
STFT Bank 2: 1.5%
LTFT Bank 2: 2.3%
Intake Manifold Data: 27.7 in.hg.
Engine: 1944 RPM
Vehicle speed: 59 mph
Mass Air Flow Sensor: 06.77 lb/min

[j cAT]

check all the vehicle grounds,,remove clean with a wire brush and reinstall..just clean all of them ,,it will take some time but this is usually whats the problem when the other componets you mentioned where changed...also you want to make sure those plug wires are correctly installed ,,,many do not take the time to install as original..

check the wires/harness for damage chaffing..

[ronaldk]

your code refers to misfire being detected on cyl 5, thats the driver side last cyl by firewall.A faulty injector cyl 5 if you have multi port injection can cause or intake manifold leak by cyl 5 etc etc etc.You changed so many things that there is not much left, I would think a bad ground could not effect only cyl 5. So are your new wires secured and away from exhaust at cyl 5 ? Just a Question I have seen it happen. Try spraying carb cleaner at intake manifold over cyl 5 see if rpm increases that is if your wires are 100% ok. Without standing over you eng its hard but those are things I would try given your info. Good luck

[FriendshipUSA]

Hi,

Thanks for the great suggestions! Sorry that I did not look at the replies until just now. I will perform the spray test and look over all the wires sometime this week after work. I have the service book and it does not specifically note where the grounds are. The electrical diagram suggests somewhere along the wiring hardness but I can’t find it.

What I did this weekend:
The first was a poor man's test to see how each cylinder was firing. With the engine at an idle I connected my timing light to each spark plug wire one at a time and looked at the flashes of lights. Cylinders 1,3,5 looked OK. Cylinders 4,6 appear to have more sparks than the others. The "additional" flashes were intermittent. Perhaps these are due to an inductive pickup from the other wires. I don't know, just guessing. Cylinder 2 looked OK as well. A better test would have been to use two timing lights. Each connected to a different cylinder and to look at the two cylinder flashes at the same time. If each cylinder is receiving the spark as expected, then, the two timing light flashes would be in unison. One flash followed by the other. If there were any missed or extra flashes, then, my eye should detect a change. Unfortunately I have only one timing light.

The second thing I did was to pull off the cap and rotor and inspect them for carbon tracks. I found none. I also blew out the distributor with compressed air. I did this to blow out any small metal contaminate (small piece of metal, wire, or solder) that could be floating around in the distributor. After the rotor and cap was installed the mystery miss remained. Oh well.

[j cAT]

Originally Posted by FriendshipUSA

Hi,

Thanks for the great suggestions! Sorry that I did not look at the replies until just now. I will perform the spray test and look over all the wires sometime this week after work. I have the service book and it does not specifically note where the grounds are. The electrical diagram suggests somewhere along the wiring hardness but I can’t find it.

What I did this weekend:
The first was a poor man's test to see how each cylinder was firing. With the engine at an idle I connected my timing light to each spark plug wire one at a time and looked at the flashes of lights. Cylinders 1,3,5 looked OK. Cylinders 4,6 appear to have more sparks than the others. The "additional" flashes were intermittent. Perhaps these are due to an inductive pickup from the other wires. I don't know, just guessing. Cylinder 2 looked OK as well. A better test would have been to use two timing lights. Each connected to a different cylinder and to look at the two cylinder flashes at the same time. If each cylinder is receiving the spark as expected, then, the two timing light flashes would be in unison. One flash followed by the other. If there were any missed or extra flashes, then, my eye should detect a change. Unfortunately I have only one timing light.

The second thing I did was to pull off the cap and rotor and inspect them for carbon tracks. I found none. I also blew out the distributor with compressed air. I did this to blow out any small metal contaminate (small piece of metal, wire, or solder) that could be floating around in the distributor. After the rotor and cap was installed the mystery miss remained. Oh well.

measure the plug wire resistance..as well as the other suggestions..

[FriendshipUSA]

Great thought! I may have some time late this week to investigate.

[2000CAYukon]

The distributor drive gear should also be inspected. It may be worn.

//2000CAYukon

[FriendshipUSA]

Thanks for the input.

When the rotor and cap was off I pushed/pulled on the distributor shaft. It felt OK and notice no play.

If I takeout the distributor what do I look for? Is it obvious?

I plan to work on the truck tomorrow and report what I found.

[2000CAYukon]

Originally Posted by FriendshipUSA

Thanks for the input.

When the rotor and cap was off I pushed/pulled on the distributor shaft. It felt OK and notice no play.

If I takeout the distributor what do I look for? Is it obvious?

I plan to work on the truck tomorrow and report what I found.

Here are some pics of a very worn gear http://www.chevytalk.org/fusionbb/sh...hp?tid/217089/

You want to look for wear and if necessary compare the drive gear with a new one at a parts store.

//2000CAYukon

[Bob B]

Hi,
Did you ever find the missing engine?

[FriendshipUSA]

Hi everyone,

The mystery miss remains.

What I did today:
1)The cyl 5 spark plug was replaced with a new Bosch 2-pole platinum plug. The old plug was a Bosch 1-pole platinum plug.
2)The cyl 5 plug wire was pulled, inspected, and measured. The wire is a Bosch Ultra Premium 7mm=opti=layer Mag Core. Its measurement reading was 10.6 Ohms. The wire was flexed to test for an intermittent problem. None was found. The resistance varied a little (1 ohm). There was no exterior damage on it. It measured and looked good.
3)The distributor cap connectors for cly 3 & 5 were inspected using a mirror. They were identical and looked good. (cly 3 was for reference to ensure cly 5 was OK).
4)The new plug and the current plug wire were installed.
5)Clys 1, 3, and 5 plug wires were dressed so that they avoided any metal including the exhaust.
6)I tried spaying carburetor cleaner around the intake but was only able to get to the top side (and not very good). The intake manifold area next to the fire wall was not accessible. (does anyone have any tricks getting to this spot?). So the full test was not done. We don’t know if a manifold leak is causing this. For the little done, the idle speed did not change.
7)I have not pulled the distributor. The photos were great and I now know what to look for.

I cleared the codes and gave it a test drive. No noticeable misses for the first ½ mile. I kept the truck under 35 mile/hr. Then the mystery misses appeared. Slowly at first, then, it got worst at a higher speeds (64 miles/hr). When I returned I performed a code scan. Code P0305, cly 5 misfire, was again set. I tested the truck several times in this manner through out the day. The same scenario each time. The truck started out OK, later started missing, and Code P0305 was reset. What is interesting is that the truck appears to have much less (none at times) missing during an idle. But this may change tomorrow knowing Murphy.

My next step looks like to pull the distributor (or spaying carb cleaning around the intake if someone can recommend a good way to do it). I will have to wait to pull the distributor until I can quickly find a replacement part. If this is the problem I can't be without the vehicle long.

Does anyone have any other possible causes? We have not ruled out a vacuum leak. Could it be an sticky valve or the injectors? Does one of these better explain the observed symptoms?

Thanks for all the help.

[jdmccright]

Another thread jogged my memory on these CPI injector modules where the poppet valves would stick. You may have to remove the "spider" injector assembly to give it a good cleaning and check for sticking injectors/poppet valves.

[FriendshipUSA]

Thanks for the suggestion.

I believe removing the "spider" injector assembly can be done without pulling the intake and should not be a problem. How do I clean it and "check for sticking injectors/poppet valves"?

I ordered a replace the distributor (used with low mileage) and will perform the swap out after it comes in.

Missing symptoms update:
The truck does well the first 1/4 to 1/2 miles after it is first started. I don't think it was missing. After about 1 mile it goes to a consistent miss (one cylinder?). If I stop the engine and restart it the missing is greatly reduced. Its "wellness" however does not last long.

The missing symptoms suggest a warm up influence and a mechanical induced coupling effect. Anyone else have a thought?

My next update will be after I performed the distributor swap out. This is likely to be late next week or the following weekend .

Thanks everyone.

[jdmccright]

You are correct in that you don't have to remove the intake manifold to access the injector assembly, just the plastic upper plenum.

There is a TSB to clean the injector assembly, but it involves some special parts to separate the fuel tank and injectors and use pressurized air to inject the GM Top Engine Cleaner (US P/N 88861802)...not a cheap setup:

http://www.justanswer.com/questions/...idle-sometimes

Before tearing into it, you might try some SeaFoam first which has worked well in removing gum, varnish, and carbon deposits.

[j cAT]

Originally Posted by FriendshipUSA

Thanks for the suggestion.

I believe removing the "spider" injector assembly can be done without pulling the intake and should not be a problem. How do I clean it and "check for sticking injectors/poppet valves"?

I ordered a replace the distributor (used with low mileage) and will perform the swap out after it comes in.

Missing symptoms update:
The truck does well the first 1/4 to 1/2 miles after it is first started. I don't think it was missing. After about 1 mile it goes to a consistent miss (one cylinder?). If I stop the engine and restart it the missing is greatly reduced. Its "wellness" however does not last long.

The missing symptoms suggest a warm up influence and a mechanical induced coupling effect. Anyone else have a thought?

My next update will be after I performed the distributor swap out. This is likely to be late next week or the following weekend .

Thanks everyone.

the misses at the higher speeds now indicates to me a fuel delivery problem.