Tracking down a Misfire!

[TurboSupra7]

Okay, since the day I bought the car (about 5 months ago) it has had a terribly annoying misfire only during light throttle and only after the car is completely warmed up (on cold days sometimes the car won't get hot enough and will run flawlessly all day). The car always idles great, but when it gets hot you can feel a very slight misfire in the idle. Car never overheats, although my coolant light is always on.
When I accelerate at full throttle, the car pulls hard and doesn't smoke and pulls all the way to redline perfectly.
When I let off the throttle and the engine decelerates, the exhaust gurgles and backfires.
Also something that probably isn't related, my BOV doesn't do anything until the car is almost warmed up. This has stayed true with two different BOVs.

So far I have:

Changed plugs and wires
Replaced every vacuum hose and rubber hose that I could find with silicone
retightened all intercooler pipe connections
Set TPS

I'm going to set the timing on my next day off, does anybody have other suggestions of things to check?

Car mods:
3inch exhaust with ractive muffler, stock turbo elbow
cone filter on stock accordian tube
HKS BOV

 

[TurboSupra7]

car is a 1990 Turbo

 

[mrnickleye]

Many times the gurggle, backfiring exhaust is due to a leak in the exhaust system. Have a friend help by litely going up & down on the throttle while you check the exhaust for leaks . Many times its helpful to use a 'mechanics stethoscope' to listen with.

How bad is the engine miss, really. It can have (as mine does) - sound like a 'tiny' missfire during idle, yet run seemingly perfect all the rest. Its a long motor, with parts that have miles on them. It wont idle perfectly smooth.

It only cost (at my shop) $35 to have whats called a "pre-smog" test. It prints out all the readings just like a real test, but its not recorded at the state level. You will be able to see how your mixture is. You can request a '2-speed idle test'. It does not have to be a dyno test (15 & 25mph under load), but if you asked me for both, I'd just charge you for the one.

The tech can give you info on the #s.

DON'T go to a 'Test Only' place, as they are NOT repair and diagnose techs, SO....they are not as smart as the techs at the smog 'repair' shops.

 

[suprabad]

I had a car (not a supra) with similar symptoms. It ran well at above idle engine speeds but had a stumble at idle once warmed up. Like you, I tried vaccuum hose replacement and all manner of tune-up techniques and parts. I also block tested it for exhaust gasses in the radiator and passed.

When I finally couldn't get the stumble out of it after trying everything my friends and I could think of I tore the engine apart and had the parts pressure tested and the block magnafluxed. What I found (and it was the only abnormality in the engine) was a hairline crack in the head at the top of one of the combustion chambers near the sparkplug hole.

The general consensus was that the hairline crack only opened up enough to leak when the motor was hot and the metal expanded. Appearantly it wasn't enough of an opening to make it miss or stumble under hard acceleration when the engine was burning more gas, but like your motor it was definitely stumbling at idle rpm's. Hope that's not your situation.

For what its worth.

 

[TurboSupra7]

well thanks for the input guys, but it isn't the idle miss that bothers me, I really can't even feel the idle miss unless I'm really paying attention.

The thing that bothers me is, say I set my cruise control on the highway going 70.
I'm not sure what RPM that would be at or if that even matters.
But cruising, the car will buck randomly. This is not consistent, sometimes it'll go a minute without a problem then start misfiring again.

But if I lay into the accelerator, the boosts comes on strong and the car pulls hard and smooth to redline w/ minimal smoking from exhaust.

The problem seems to get worse with higher temperatures.

 

[TurboSupra7]

I'm also going to add a few extra grounding points for the electrical system, set timing, replace timing belt, replace serpentine belt.

Any other suggestions?