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

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

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..

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

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

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

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

//2000CAYukon

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.

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/showtopic.php?tid/217089/

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

//2000CAYukon

Hi,
Did you ever find the missing engine?

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.

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.

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.

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/3coa-gmc-jimmy-stalls-idle-sometimes

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

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.

Great information!

I will proceed with the distributor replacement as it is already bought. It is good to know that the fuel system is the likely cause and what my next step is. I am very pleased no one mentioned the valves as a possible candidate.

Great link! I will do some research in obtaining/borrowing the needed tools or may a shop do the work.

Bosch ignition parts have always given me poor performance on American vehicles. Stick to A-C Delco. Initial cost may be more, but service life more than makes up for it.

All,

The mystery miss remains.

I replaced the distributor. (The old one was OK). I also replaced the injector with a new GM/Delco Multi-port injector.

Injector replacement: My internet research and talking with a mechanic confirmed the original injector were prone to sticking / clogging poppets (thanks jdmccright (http://www.automotiveforums.com/vbulletin/member.php?u=437300) for your post). The missing symptoms I am experiencing was similar to those reported by others. The new injector is a GM / Delco direct replacement and is designed with multi-port injectors. I purchased the injector, seal kit, and injector bracket. The new bracket was not necessary as the old bracket worked. The web site is: http://www.gmpartsdirect.com (http://www.gmpartsdirect.com/). My part order was:

Part no:...........Description:.......................C ost
12568332.........4.3L Multi-port injector.......$181.14
17113215.........Seal Kit.............................$24.91
88894355.........Bracket.......................... ..$23.55
Handling & Shipping...................................$48.22
Total............................................. ......$277.82

Replacing the injector took about 3 hours.

Trouble Codes: I purchased a Innova 3140 code reader. It diagnosed Code P0305, cyl 5 misfire detected. However, when it is used in the GM/Isuzu Enhanced modes the code it diagnose is P0300, Random Misfire Detected. It diagnose this for each of enhanced mode (MI1, Current DTC, Fail Since Clear DTC, and History DTC).

For on all the inputs received thus far, the items not addressed or not completely addressed are:
1) the electrical grounds were not confirm.
2) The vacuum leak test was not thorough to confirm this is not a problem
3) There is a question on the Bosh plugs used.

I appreciate any other suggests.

I am considering going to a mechanic on Monday (October 11) as I am off that day and can do without the truck.

The mystery miss is almost solved!

I was not able to take the truck to a mechanic last week. However, I opted to expand on the vacuum leak suggestion provided by ronaldk (http://www.automotiveforums.com/vbulletin/member.php?u=619371) on September 9. “… Try spraying carb cleaner at intake manifold over cyl 5 see if rpm increases …”.

It appears the mystery miss is caused by vacuum leaks, at least for the most part. My inspection found 3 leaks. Two were significant. I believe there is another smaller one will continue looking it. This is what I found:
1) The rubber seal on left valve cover used to connect a hose to the aircleaner assembly. (An identical seal is on the right valve cover. It is used for the PCV valve). The seal was damage causing a loose fit.
2) The plastic 90 degree tube on top of the manifold used to connect the brake’s vacuum booster hose. The o-ring was broken.
3) The front clip used to secure the aircleaner plastic piece to the throttle body was missing. Without this there is a potential for a vacuum leak.

I test drove the truck after the repairs. What a difference! The missing for the most parts is gone. A little remains. It is intermittent and occurs between 1500 – 2000 rmps. I don't notice this but the engine lights flashes stating the missing is occurring. I will continue to chase this when the weather permits.

.

Been following this since you 1st posted, so I'll make some observations.

1. Up & down play on the rotor is needed (guessing .002-.010). No side to side play and no gentle rotational play allowed.

2. Hose from the air cleaner to the valve cover is not considered a vacuum leak. (PCV system) Have you replaced the PCV valve?

3. A P0300 code is a generic missfire code. Stick with the mode that gives you the specific cylinder missfire code. (P0305).

4. Bosh products (as mentioned) are suspect also.

Hi,
With regards to your suggestions:

1. Up & down play on the rotor is needed (guessing .002-.010). No side to side play and no gentle rotational play allowed.

Ans: A small amount of up & down play is observed. I did not measure it. The side to side play is tight. The gear wear was none to very little. It was inspected before it was installed.


2. Hose from the air cleaner to the valve cover is not considered a vacuum leak. (PCV system) Have you replaced the PCV valve?

Ans: The hose connects the left valve cover and to the plastic piece on top of the throttle body. If I remove the hose the truck won't run. The valve cover gasket seal was damaged and if the hose shifts a little it would leak. So, I replaced the seal. I also decided to replaced the right gasket seal and the PVC valve. I don't know how the truck with the new PCV valve will work if I pulled off the left tube. A test for tomorrow.


3. A P0300 code is a generic missfire code. Stick with the mode that gives you the specific cylinder missfire code. (P0305).

Ans: The old and new code reader states P0305. The new code reader has an enhancement mode for GMC/Isuzu. In this mode it states P0300. I replaced all the electrical components, all the plug wires twice, and the cly 5 plug twice (two different Bosh plugs).


4. Bosh products (as mentioned) are suspect also.

Ans: My plan is to look for more vacuum leaks first. I will consider replacing the plugs. jonnik suggested A-C Delco. What do you suggest?

.

i probably didnt see it in the posts, but did you mention if you re-checked your spark plug in the suspect cylinder? my 4.3 was missing so i replaced the plugs but it shortly began missing again and i found that one plug was already fouled up after only a couple weeks because i must have excessive blow-by in that cylinder. i just wire brush it every time it starts acting up. lol it might be time for a rebuild pretty soon

Missing is about the same.

Update on my activities:
1) I looked for some additional vacuum leaks using carb cleaner. Fond none thus far.
2) I replaced all the plugs with AC-Delco. The old plugs looked ok and no one was fouled up. So I hope we don't have a rebuild in the near future.
3) The test drive: The missing is about the same and intermittent. It started when the truck was about 1 mile from the house and the rpm was about 1500-2000. The engine light blinks on and off suggesting a misfire.
4) I performed the test suggested a few days ago. The tube connecting the passenger's side valve cover to the plastic piece on top of the throttle body was disconnected. The engine performed about the same as if the tube was not pulled off. I put my thumb over the tube connected to the left valve cover and noticed a small vacuum pull. The results were very different before the rubber seals and the PVC valve (on the right side valve cover) were replaced. When the left side tube was pulled the engine died and I was unable to keep it running. Perhaps I had a bad PVC?

Observation: the missing is intermittent and was present after the new plugs were installed. The commute to and from work suggest the missing occurring for several miles then not for the next several miles. The missing has also occur at idle. If it is present at idle, then, it stays.

My plans:
1) Switch the plug wires between 3 & 5 and see if cly 3 misfires (code P0303?)
2) Look for more vacuum leaks.

I was looking at my 96 service manual earlier and noticed something odd with the 4.3 injector wiring, the power for cylinders 1,2,3,4, and 6 are spliced in one spot and the splice for cylinder 5 is in another according to the wiring diagram, so it may be possible that you have a suspect splice in the injector wiring

I was looking at my 96 service manual earlier and noticed something odd with the 4.3 injector wiring, the power for cylinders 1,2,3,4, and 6 are spliced in one spot and the splice for cylinder 5 is in another according to the wiring diagram, so it may be possible that you have a suspect splice in the injector wiring


Jeremy-WI,

I would like to look into this. Can you tell me where this is in the service manual? Does it show a different ground for cly 5?

The grounds are all separate, so the pcm can control injectors individually, its in book 2, page 6E-25. You could just pull the connector off for the injectors, its just behind the throttle body and check for power at the pins with the ignition on, you should have 12v at pins B, D, E, H, J, and L

The grounds are all separate, so the pcm can control injectors individually, its in book 2, page 6E-25. You could just pull the connector off for the injectors, its just behind the throttle body and check for power at the pins with the ignition on, you should have 12v at pins B, D, E, H, J, and L


Thanks Jeremy-WI,

Sorry for not getting back sooner. Due to other commitments this week, I was unable to address this Email nor work of the truck.

The information is very helpful. Today, Saturday, is raining and so this weekend is likely a bust with the exception of digesting the shop manual material you referenced. The missing is intermittent, same code P0305. It occurs at an idle for time to time but now has become more frequent. If I get it into this state it should help me diagnosis the problem.

I had the same problem with my 1999, 5.7 Tahoe. Cyl. 5 misfire. Tried changing all electrical parts. Sprayed all over intake. Changed injector spider. Finally found an internal vaccum leak. When I pulled off the oil filler cap with the engine running the RPM's went up rapidly. Intake manifold gasket was bad. Replace gasket and it fixed all problems

Old post I realize, but were the #3 and #5 plug wires ever switched out? Seems I remember that wires should have a max of 10k and any more resistance the plug wire is shot. (thought I read the OP tested resistance on #5 wire and came up with 10.6k ohms)

I also would question bosch ignition components as they do not play well with GM motors of any type.

One more question, how did the tip of the #5 plug look when you pulled it out? Wet with gas? Oil build up causing missfire? (bad valve guide), or was it clean?

I usually start out on issues like this by doing a compression test on all cylinders. Maybe give that a shot. Maybe there is a burnt valve?

A note about Bosch Pt spark plugs. The way they are designed with the center electrode surrounded by ceramic except the very Pt tip does not allow for gap adjustment after they become worn. The tip erodes away down into the ceramic, so your feeler gauge will not give you a true gap distance between the electrodes.

A-C Delco plugs are the consensus and I have no doubts about their relibility, but I personally have liked the Autolite Pt plugs.