'miss' when on throttle/shudders when coasting in gear

[ssstuds]

Hey guys, I've been playing with this car for a while now off and on (I only get a chance to work on it every couple weekends). I'm kinda stumped as to what it might be. It is a 1992 TSi AWD.

Two main problems:
1. When I hold my throttle at or above 1500rpms every second or two it will drop 1-200 rpm's and then jump back to 1500 for 2 seconds and then drop and then repeat. If I hold the rpm's below 1500rpm everything behaves normal, rpms stay where I hold the throttle. It is only @ or above 1500 when I get the stutter.

2. Most likely related to #1, while coasting (doesn't matter what gear, more noticeable in the lower gears) it feels as if the engine isn't getting enough gas and the car begins to shudder. Under boost or normal acceleration the car is perfect. Never feels bogged down, zero knock, etc. It becomes more jerky in the lower gears when I let off the gas, this is to be expected due to the lower gearing.

Background of the car:
I bought it about a year and a half ago and have been slowly replacing a lot of mechanical stuff on it. Redid the suspension, brakes, replaced the turbo, exhaust manifold, etc. There is a brand new O2 sensor, brand new ISC, new plugs/wires. I cleaned out the throttle body, while doing that I tested the TPS and it check out within spec. After re-assembling the TB I set the base idle speed, then base timing. From watching my pocket logger the new O2 sensor is behaving correctly. I have an AFPR in place of the stock one, fuel pressure is set to stock values. Vacuum on my boost gage is in spec.

The coasting issue almost feels like the ecu is cutting fuel, I haven't really tried watching the pocket logger that much with this problem.

I'm interested in any suggestions anyone may have. I'm going to try swapping out the coil pack this weekend and see what happens with that.

Thanks for any input everyone.

 

[ssstuds]

I got some more info if this might help someone out in giving me any ideas.

1. I removed the MBC that was on the car and put a spare OEM one back in. Re-routed all of the vacuum lines related to the MAS, BOV, and turbo to their stock location/splits (the guy had some lines plugged w/ the MBC, I just wanted to rule any of that stuff out).

2. Tested out the idle switch (the 1 prong switch on the TB that tells the ecu if you are idling) with a multimeter and it opens and closes correctly. Also watched the pocketlogger and the switch is opening and closing correctly when I give it gas.

3. This only happens when the car is warmed up for about a minute. It's ~40 deg F here, when the car is cold and I hold the rpm's up, it doesn't miss at all. It is only when the car is warm that this is happening.

4. Blocked off the EGR.

Does anyone have any ideas at all regarding which sensor could cause something like this? I can't drive the car like this, so it's just sitting there. I really want to have it running by the time is snows.

Does anyone have any first hand experience taking a car to the dealer for diagnostics? What do they charge and all that?

 

[herm1357]

about the coasting issue: if your driving at crusing speeds with very light acceleration or none at all your problem comes from the egr valve being blocked off. try unblocking it cleaning it out hooking up the lines to the thermal vacuum valve. you will see no horsepower gains from it being blocked, the egr valve actually reduces combustion chamber temps

 

[ssstuds]

I actually had the problem before I blocked off the EGR. I did it to try to take the EGR system out of the mix in case something in there was causing the problem. A few people suggested trying to throw in a block off plate just to see if that helped.

Thanks for the reply, let me know if you have any other ideas.

 

[herm1357]

i know you said that you checked the tps already, but it wouldnt hurt to double check it becasue it sounds like it could be the tps, make sure its cracked and has the metal clip stilll

 

[ssstuds]

OK, figured it out.
Actually, I didn't figure it out but found something to fix it.

At some point in time I had swapped the OEM ecu with one from a 93 parts car I had (it was a TSi awd, no abs, 5 sp). The only difference between the two cars was that one was a 92 and the 93 didn't have ABS, the 92 did. The caps on both ecu's were good, the board on the 93 actually looks like it is only a few years old.

So anyway I put the 92 ECU back in and my problems are gone. Are the abs/ non-abs ecu's different? I don't know what could have been causing the problem but this was the last thing that I was going to try before taking it to a dealer. I'm really happy I tried this out.

Thanks for all the help guys, it is much appreciated.

Jeff