2002 Chevy Express Van 1500

Cirp73 wrote: Maybe the schematics I have are not thorough.

Or maybe you didn't look at the power distribution diagram for the ECM-I fuse? OE diagram shows it being fed by the same, hot in start and run, circuit that feeds the airbags.

I see that now Matt.
Thanks for pointing that out.

So I got the van back from the dealer today. Their diagnosis was this: “More than likely the intake valves.” That is their speculation based on the P0300 code, which was the only code in the history, and the mileage.
They took it out for a test drive and no engine light came on. However they said they could “slightly feel the miss and they were basing their speculation based on the number of Express vans coming thru the garage over the years, with similar problems. So at this point I still have nothing conclusive. They were mighty nice by not charging me anything for their speculation. So now I’m wonder if the valves could really be the issue. Anyone have suggestions how to check intake valves? Is that even possible? Is the P0200 code still worth chasing? Any suggests would be greatly appreciated.

An in cylinder pressure waveform would be helpfull

Cirp73 wrote: That is their speculation based on the P0300 code, which was the only code in the history, and the mileage.

To be clear, P0200 was NOT stored along with the P0300?

If so, then I take it back. :silly: P0200 and P0300 may be two separate issues.

So now I’m wonder if the valves could really be the issue. Anyone have suggestions how to check intake valves? Is that even possible? Is the P0200 code still worth chasing? Any suggests would be greatly appreciated.

If you really wanted to prove or disprove a valve sealing issue, compression and leak down testing is the next move. I saw that you've already done compression testing - do you recall what the values were?

Tyler...also right now the only code showing is the P0300. I cleared the codes and after ripping apart and reassembly, the only code firing is p0300. MIL flashing when code thrown. Seems as though the p0200 is set when the light comes on and stays on and that hasn’t happened yet since I cleared the codes. I just ran the van today(mostly highway) back and forth total of 114miles. No MIL lite but heavily miss firing in the course of travel.

When does the misfiring occur, exactly? Or, when do the #3 miss counts start counting up heavily? Idle, cruise, heavy load?

Tyler.....That is correct. No P0200 stored as of yet, but I can guarantee it will set eventually.
I was thinking along those same lines too, that there maybe 2 separate issues.

The values of compression were as follows:
(Cyl#1...185) (Cyl#2...180) (Cyl#3...170) (Cyl#4..185) (Cyl#5..190) (Cyl#6..195)

The miss-firing generally occurs and is noticeable at cruise speeds.

Thinking back to when I tore down the upper intake plenum; The throttle body and upper intake were all gummed up.
The pedal was sticking for the longest time, and the mechanics I would take it to could never give me a reason as to why.
So when I cleaned all the above mentioned out, and put everything back together, the pedal did not stick anymore.
So now am wondering about the valves being heavily Gummed and causing them to stick? could that be a possible cause of P0300?

The P0200 still has be baffeld!!

Thanks for the input Hardtop.
Don't think I have the necessary tools to perform what you suggest would be helpful
It sounds like I need specialized tools for that?
I'm just a WW/DIYer.

So what I’m reading about compression on the Chevy 4.3l motor...100 psi is the minimum for each cylinder. So my next question would be are the values I recorded when doing the compression test a bit high or high in general? I’m gonna try to get a leak down tester today and do the leakdown test next. I’ll post results when finished.

Thanks again to everyone for commenting. I’ve learned a lot within the week I been on the site/forum.

Cirp73 wrote: So what I’m reading about compression on the Chevy 4.3l motor...100 psi is the minimum for each cylinder. So my next question would be are the values I recorded when doing the compression test a bit high or high in general? I’m gonna try to get a leak down tester today and do the leakdown test next. I’ll post results when finished.

Thanks again to everyone for commenting. I’ve learned a lot within the week I been on the site/forum.

I looked in SI and couldn't find a compression specification for the 4.3L. But the 100 PSI minimum thing is more of a general rule-of-thumb, IMO. I don't care for it very much. :silly: It's entirely possible to have over 100 PSI compression and still have misfiring.

I don't know that I'd spend time leak down testing every cylinder? Just #3. It's the one flagging miss counts, and it also happens to have the lowest compression readings. Maybe do one other cylinder to have a known good result if you're unsure.

The miss-firing generally occurs and is noticeable at cruise speeds.

That makes me go back to ignition and fuel. Leaking valves generally don't create heavy misfiring under cruise or load condition, just at idle. A broken valve spring or a valve sticking in the guide, maybe.

Finally got around to doing a cylinder leak down test on the following cylinders and here are the results:
1.......90/76
2.......90/78
3.......92/68
4.......90/78

Didn’t test cylinders 5&6

Noticed a little bit of hiss out of dipstick and oil filler cap on all cylinders tested. Air compressor kept kicking on to refill tank. Didn’t notice leaks anywhere else, so I believe I was at TDC for each cylinder under test. Cylinders 2&3 I had to hold the crankshaft pulley in place with a breaker bar while the chamber was being pressurized because the piston would slip and turn after around 30-40psi was applied. Is this normal? Didn’t have to hold on cylinders 1&4.

May end up performing this same test on my Chevy Silverado for comparison.

May also retest as I was pressed for time.

Also found what appeared to be a very frail/partially broken bare ground wire attached to the stud which holds down the thermostat housing(gooseneck).
There were 2 other wires attached to this stud but they were ok. I couldn’t find anything on the schematics I have to Indicate it’s purpose. I followed it back thru the wiring harness as far as I could without removing all the tape and loom. It heads toward the fuse center. I spliced in a new piece of insulated 18awg and reconnected to the stud. Anyone who could clarify what this wire is for would be helpful.
I looked at the ground distribution diagrams and couldn’t find anything coming and off of the thermostat housing. I use (auto repair source motor driven schematics)

Update:::6-5-2021
I now believe the actual issue is with the transmission and not the engine.
Since I don’t have the proper tools to diagnose transmission, I will be taking it to a shop for further investigation. Since fixing the broken ground wire I previously mentioned, the check engine light nor are there any codes stored in memory.

Sorry for the late reply but yes gummed up valves would cause a misfire. Cleaning the valves would be a good start in your case. What makes you think it's a problem with the tranny?

Also misfires at cruise speed typically lean towards coils going bad (no misfire when cold started but once it warms up or under a load like at cruising speed it will act up much like an injector as well)

I have a 2000 4.3 GMC Safari van and have been changing caps & plugs etc for several yrs also which where probably not the true issue as well. Sorry I don't have any advice because mine has been no start for 2 mo. I just wanted to say I feel your frustration. I hope you don't mind. I've seen online that Express, Astro, Safari 4.3 Vortec have similar mystery problems starting with looking at the cap and rotor and expanding from there. No one ever seems to post what the solutions where. SD training and Tyler on this forum has been a huge resource for me lately, I'm a new member. Also I stopped driving mine in OD because at cruising speed it would upshift and downshift really rough and jerky Good Luck!. 

Thanks for the reply Urso6566!
That’s exactly what mine has been doing all along. I thought it was engine related, but I’m leaning more toward tranny now. Seems to be in the 3-4 shift when the van starts to buck. As for the engine. It definitely had its share of problems. Since I found and repaired a bad ground in the harness, I have not had a miss fire code(p300) or P(200) reappear.

Thanks for the reply hardtopdr2.
The reason I think it’s the tranny is because of the jerking at cruise speed and it seems to be happening in the 3-4 shift. Or vice versa. 1-2 gears seem good shift. I changed coil pack and a whole slew of other items. I will eventually get around to cleaning the valves, but I need the tranny diagnosed first. Stay tuned.
Thanks

If you have a scanner with graphing view look at the trans data for TCC solenoid, 3/4 shift solenoid and line pressure data. Also what trans do you have in the van ? 4l80 or what?

4L60E is whats in it.

did you use the updated spider injectors?

I quickly scanned through this post. I did not notice anyone mention Cam Retard. (i am not calling you names, this is really what the PID is called.) It needs to be within 5 degrees of 0. 0 or slightly positive is desired. If its off by 7 Degrees it will miss fire. its the most common intermitten misfire with the GM Flat top Distributor engines. look at your scanner data. To set it you turn the distributor like you are setting the ignition timing on a carb engine. Some are not Adjustable. Most Aftermarket Distributors that are not adjustable will be off brand New. I have never gotten one from Autozone that did not have to be sent back. When your setting it, you have to rev the engine above 2k rpm for the PCM to register it. (snap throttle) As for the odd OD shifting. If seen 3 of these vans that the MAF was dirty/faulty that would cause this. These vans use the Load Percentage from the PCM to determine Shift Points. The biggest contributor to the Load percentage is the MAF.

Yes I replaced spider with updated