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

[FriendshipUSA]

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.

[jonnik]

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.

[FriendshipUSA]

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 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. 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.

[FriendshipUSA]

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 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.

.

[777stickman]

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.

[FriendshipUSA]

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?

.

[jveik]

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

[FriendshipUSA]

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.

[Jeremy-WI]

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

[FriendshipUSA]

Originally Posted by Jeremy-WI

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?

[Jeremy-WI]

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

[FriendshipUSA]

Originally Posted by Jeremy-WI

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.

[CARAP]

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

[tblake]

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?

[jdmccright]

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.