2002 Chevy Express Van 1500

V-6 4.3L 171k miles.
P0300 and P0200 DTC.
Intermittent heavy miss fire at load.(typically 30mph & upward while holding pedal steady)
Only way to stop the miss fire is by letting off or pressing the pedal more.
No apparent problem at idle

History:
I’ve been chasing this random multiple miss fire for more than a year now. I believe it’s been occurring much longer than that. No one seems to be able to get to the bottom of this, so I took matters into my own hands around January 2020 after the van broke down on me when traveling from Pittsburgh, Pa to Wheeling, Wv. I had the van towed back. Prior mechanics would always put new cap and rotor on distributor(Yearly), so I started there and WALA!! It started. While changing the cap and rotor, I also noticed that the back torx screw wasn’t tightening down. The bottom of the distributor where the cap attaches is made of plastic and was stripped/cracked. Someone overtightened/stripped it out along the way. So for the interim(until I found a Bracket that addressed that specific issue) I used 2 ty wraps to secure the cap. Everything was fine for a day or so and then the miss was back. So I revisited the distributor thinking moisture was getting in. Nope! All was looking good with cap and rotor. This was about the time I started watching you tube videos and started throwing parts galore at it in an attempt to remedy the miss fire.

Fast Forward:
Here is a list of what I’ve replaced either as a guess or because the part was OE and at the end of its life cycle.
▪️Replaced crushed/split vacuum hose to pvc valve.
▪️Complete new distributor which came with new cap and rotor. I changed this because of the broken plastic.
▪️Replaced plugs and wires
▪️Replaced original ignition Coil. (This thing would of lite up the night sky had the engine cover not been on).
▪️Replaced downstream O2 sensor after receiving advice from someone. This was a nightmare because the threads were stripped, so I have to remove the exhaust to fix threads.
▪️Smoke(homemade)tested to look for additional vacuum leaks and boy were there leaks around the upper intake plenum.
▪️Cleaned throttle body & 18years of carbon deposit build up.
▪️Inspected and replaced fuel injection spider since I was down this far and found tip on cylinder 2 poppet broken (read about issues with OE spider)
▪️Replaced gaskets on all of the above.
▪️Replaced MAP Sensor since the rubber bushing was deteriorated.
▪️Checked TPS values with ohmmeter. Ok
▪️Retested leaks successfully with smoke machine.
▪️Replaced fuel filter.
▪️Replaced ignition control module
▪️Checked compression. All good here.
▪️Changed plugs from iridium to double platinum.(watched you tube video about 4.3L not liking iridium plugs) lesson learned here. Be careful what you watch!!
▪️Replaced ECM/PCM.
▪️And finally inspected wiring harness for opens, shorts, grounds. Found schematics through MOTOR driven auto repair source.

At this point Im super frustrated and have made an appointment with dealer for diagnostics only. Wish I had found this site sooner. I’m hoping the dealer can properly diagnose, but have my doubts since I was having a similar issue years ago and a different dealer recommended a (Major) tuneup. This my work van, so that being said, it is detrimental that I have a reliable means of transportation. Although it is running at this moment, the more I run it, the more it seems the miss is getting worse.

What I haven’t tested for:

▪️fuel pressures since I don’t believe I have the proper tools to command injectors.

What scanner I’ve been using but am not sure if I’m using/interpreting properly.
AUTEL MaxiDiag.

You need to focus on the p0200 code. It’s a injector control code. Hopefully someone can post a diagram of the fuel injector circuit. I bet the problem is in the fuse box under hood.

Do you have an amp clamp and scope to current ramp the injectors? What do the history misfires show what cylinders have the highest counts

Cheryl wrote: I bet the problem is in the fuse box under hood.

Nope! I ain't taking that bet. :silly:

Knowing the History Misfire counts would be very helpful. I can't say if your MaxiDiag will read those or not.

If the P0300 and P0200 reset at the same time, then Cheryl is absolutely right. Follow the P0200, and you'll find your miss.

Tyler...my scanner does read those and cyl 3 records the highest count. Cyl 2 is a distant 2nd but it too shows miss fire counts. When watching the live miss firing counts all cylinders are recording misses.

Cheryl...I do have an amp clamp but not a scope. Furthermore, I had the under hood fuse box completely apart and there is no special circuitry inside, so if your thinking along the lines of a cracked solder joint, there is nothing there. Just terminal ends underneath and the fuses up top that connect it all together. All seems solid there. Also I do have the circuit diagrams..one thing I do find peculiar is the following scenario. Key is off, injector harness is detached from atop the spider. Test light is connected to battery positive. Lamp lights up when probing any of the b+ of injector wiring. When key is on, light goes out. I litterally ripped apart the wiring harness and traced the wires back to the under hood fusebox.(ECM1). Further more I found that the injector circuits b+ is connected to the airbag module when the ECM FUSE is left in place. So I went even further as to trace the wires thru the firewall into to the cabin. I basically disconnected everything except for the heater wiring inside the cabin and still the light remained on as if there was a short to ground. It was only when I pulled the inside fuse out or disconnected the airbag module itself the light went out. I can’t find, on the schematics I have where it would be picking up a ground thru the airbag module. After I put everything back together, I thought by maybe by pulling the airbag fuse my miss would go away. NOPE!! Maybe the schematics I have are not thorough.

I really do appreciate both of your responses and input.

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.

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.