Misfire under load, increase with engine warmth and run time

**Wakingcrow** · 08-12-2020

Having a hard time with this one. Cold, runs fine for a few minutes, plenty of power. Warm, misfires while accelerating at any speed. Hot, misfires increasingly. After a few hours it's running on one or two pistons. Runs fine after cool off.
Recently, I replaced vacuum hoses and fuel filter. Swapped the 2 map sensors to see if it did anything different. Didn't.
Code says 300 random engine misfire. Autozone printout said ASE technicians would recommend reprogram PCM. I don't have a tuner. Does anyone know what would be most likely or what to test first?
Thanks.

**bandook** · 08-13-2020

ICM or a couple bad coils maybe.

**markpa** · 08-14-2020

^ what he said.

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**BrandonHall10** · 08-14-2020

A while back I found a pretty good step by step tutorial for diagnosing a random misfire on a 3800.

Part 1 -How to Diagnose Misfire Codes P0300-P0306 (GM 3.8L)

At the end of the day my car needed a CASE relearn.

Might cost you a pizza or 6 pack if you know the right guys.

**ThatOldCarSmell** · 08-14-2020

FWIW, my experience has always been that a misfire increasing as the vehicle warms & goes away as the motor cools is usually a cracked solder joint somewhere (electrical failure). I'd Think ICM more so than coils since it's affecting all but 2 cylinders, but I'd follow the guide above before beginning to replace parts.

**Wakingcrow** · 08-14-2020

After posting I saw another misfire thread similar that it was coil packs. That would be easiest. I'll look at the guide first.
Thanks all.

**Wakingcrow** · 09-01-2020

OK so after the cylinder balance test, I got every cylinder worked, or as the guide says "More specifically, this test result lets you know that the fuel injector, spark plug, spark plug wire, Coil Pack is OK for that specific cylinder."
It implied the next step is to check the ICM, but doesn't say how, so I got a much younger one used and nothing changed. A case study had the ground as a problem but not sure how to test that . Then test compression I guess?
I feel like it's electrical problem but I don't have a vacuum tester or compression tester anyway.

**Wakingcrow** · 09-01-2020

I forgot I also put on new coils before that, but the problem keeps getting worse, it doesn't even need to be warm, just no acceleration and multiple misfire when pushing pedal.

**ThatOldCarSmell** · 09-01-2020

Not saying to run out and buy one until you have a way to check it for sure, but I have heard of a bad/dirty maf causing a random misfire too. Maybe try unhooking the connector at the maf, and then starting it. it will struggle at first, but as soon as it realizes there is no maf signal, it may smooth out. In other words, if it runs better with the maf unhooked (after settling), then your maf is likely the culprit. and that's another free & easy type of test.

**BrandonHall10** · 09-01-2020

What do your fuel trims look like?

**Wakingcrow** · 09-01-2020

Yeah I was trying to avoid the map sensor purchase. I swapped the map and maf since they're the same, and no change so I almost ruled it out unless they're both bad. That's the first thing I did. I couldn't find matching at junkyard because no supercharged there.
I don't know what my fuel trim looks like. How do I tell?

**ThatOldCarSmell** · 09-02-2020

Originally Posted by Wakingcrow

Yeah I was trying to avoid the map sensor purchase. I swapped the map and maf since they're the same, and no change so I almost ruled it out unless they're both bad. That's the first thing I did. I couldn't find matching at junkyard because no supercharged there.

What year is your car? The map and MAF are not the same sensor. The MAF has a protrusion that goes 2/3 of the way into the throttle body on most if not all of the pre- 04 3800's and should be the same maf, supercharged or not. The map is not only shaped differently, it functions differently. The quick way to test one with out a code reader/multi-meter, is how I indicated in my last post.

**BillBoost37** · 09-02-2020

Want to know the worlds easiest maf test? Start car, use your fingernail to tap on the top of the maf, do you notice a change in motor when you are tapping? Most of the time when I've come across a bad maf in that style setup, it reacts to a little tap.

**ThatOldCarSmell** · 09-02-2020

Originally Posted by BillBoost37

Want to know the worlds easiest maf test? Start car, use your fingernail to tap on the top of the maf, do you notice a change in motor when you are tapping? Most of the time when I've come across a bad maf in that style setup, it reacts to a little tap.

That too. I feel like I should spot check myself here though- do you know anything about the map & the maf being interchangeable? maybe I've just missed something along the way.

**Wakingcrow** · 09-02-2020

The two sensors are the same on my 04. Stock sensors, same number stamped on top, look identical.
When the maf is disconnected, the rpm fluctuates wildly and sometimes stalls. It never evens out.
Tapping on it doesn't make difference.
At first I thought it was maf related but I can't prove it. Need to figure out how to vac test that line

**mguzzo** · 09-02-2020

MAF sensor

MAP sensor

**BillBoost37** · 09-02-2020

The two sensors are as noted with the above pictures totally different.

There are two MAP sensors on the SC cars located on a bracket behind the SC. And yes they are identical. MAF is in front of or in throttle body based on year.

**Wakingcrow** · 09-03-2020

I was misinformed elsewhere on this site. The two maps are what I swapped.
I tapped the maf and no change I disconnected it and engine hesitated then kept rolling. I pulled it and blew it with an air duster cause I don't have the cleaner. But it looked fine.
If the maf is a prime suspect I can get one with half the miles from the yard for like 7 bucks.

**mguzzo** · 09-04-2020

you can gently clean the MAF sensor wires with a foam Q tip and isopropyl. Use a light touch to remove any debris, don't break the wires!

**FordMan77** · 09-05-2020

There is also a specified MAF cleaning spray from CRC. I've used this on multiple cars and have never had an issue and it avoids touching the MAF wires with anything. Just spray it down, let it dry and reinstall.