Thread: Annoying idle misfire

I've swapped coils, wires, plugs, ICM, oxygen sensor, and I've checked all the vacuum connections, injector to intake points for vacuum leaks, made sure throttle body was tight, even re-built all the vacuum lines... Compression checked good, I can't remember the numbers but they were all close to each other. Haven't done a leak down test yet, that's on the list. Disconnected MAF, no change. Cleaned idle air valve, no change. Haven't swapped injectors yet either. The misfire does seem to get better on warmer days. Still there, but not as bad.

It's a top-swapped L36. The misfire is usually on 3 and 4 at idle speed, but recently it's just a "random multiple misfire" code now. The misfire goes away as soon as you give it gas. Fuel trims are maxed out whenever the car is sitting still. As soon as you start moving, the trims settle back down closer to 0. It sounds characteristic to a vacuum leak, but I've tried everything. Even tried blocking the line to the booster and the HVAC line - no change. The car runs great at speed, no misfires or anything, KR hardly ever even blips when I merge into traffic or leave stop lights. Recently added a PLOG because my front manifold was cracked, I had hoped maybe that was causing the O2 to read lean, but since replacing that, no change. I noticed a slight blue tint to the exhaust, so I cleaned the PCV valve (the check valve was sticky before I cleaned it), and that seemed to help the blueish smoke. Also, at idle, the O2 sensors are flat-lined around 45mv, no change up and down like normal. When I'm cruising, they bounce up and down like normal. Again, I've swapped the o2 sensor pre-cat, no change before or after.

I'm a step away from just building a new engine and throwing it in with the hopes of it running better. Any ideas before I start that?

Side note, on the chance it's a tune issue, what MAF table should be used for a top-swap? I've been using the L36 tables, slightly modified. After all this, is THAT my problem?

try a new icm, or a used one. seems you hit every other base.

Thanks for the tip, but that was one of the first things I tried, with coils and wires.

give the wires to the icm a good once over looking for damage, as well as the crank sensor wire. make sure the icm ground is on and tight too.

if you used a used icm theres always a chance its also bad.

I'll double-triple check that, thanks. Clean is good, so we'll see.

Ok, I went out and unplugged the MAF sensor while it was idling, and watching the scanner, the fuel trims started going down and finally settled in around 0. When I plugged it back in, the trims started to spike again and quickly hit +20/+16. Bad MAF? Wouldn't that happen while cruising too if it was bad?

thats not how to do the maf unplug trick, you only pissed off your car unplugging it while running.


car off, un plug it, then fire it up, this will put it in closed loop, if it runs better the maf is bad.

Gotcha. I'll give that a shot in a bit.

if you pop the hood then shift from P to R then to D, then back to R, and watch the engine. if its moving a lot when shifted in and out of gear the mounts are bad too.

Ok, engine off, unplug MAF, start engine. Trims settled back down, ST was around 0 and LT was around +6. Kill engine, plug MAF back in, start engine. Trims maxed out, LT at 16, ST at 20.

Unplugging the MAF puts it into speed density and it uses the VE table that is in the computer.

I think you may indeed have a maf problem if when it was unplugged while running the fuel trims leveled out and was all normal but when plugged back in they went all whacky. If you have another car with a 3800, swap mafs.

If you are running a L36 throttle body, you will want that MAF table.

And make sure the injector data is correct if you swapped to GTP injectors, which I assume you must have.

I'm running a L67 throttle body, and I'm using the GTP injectors. That all has been changed in the BIN. The MAF came out of a '99GT, but aren't they all the same from '99+? Thought I heard that somewhere.

The MAF sensor is the same, but the size of the throttle bodys are different. I will admit I have never done a top swap tune before, but I would have put in a maf table for a L67 of the same year in the tune to start off with, only because of the TB you are running.

Those 02 readings shouldnt be flat lining like you say, should be bouncing up and down all the time. Do the o2's start switching more when the MAF is unplugged??

I changed the MAF table today to stock GTP, and it seems the misfire is a little better; it seems to be running a little more smoothly. And yes, the O2s started cycling when the MAF was unplugged. When MAF plugged in, they only cycle with some throttle above ~1200RPM, and go flat in any off-idle situation.