Misfiring intermittently during highway cruise

[steel_3d]

I'm a little bummed at the moment since my brand new engine is starting to give me the same symptoms as the previous one did when it blew the head gasket.

Basically I'll be driving on a flat highway at a constant cruise, or even downhill, and suddenly the car starts stuttering, misfiring. It's a very regular misfire like one cylinder stops firing completely. Letting off the throttle or giving it more gass, or playing with it in any way doesn't help, until I push the clutch in and let the rpm's drop to idle. Then I can go again for 5-10-15 seconds before I have to repeat. If I drive below 70 it seems to do a lot better and exhibit the problem much more rarely. If I go above 3000rpm the misfiring becomes more and more frequent. This only starts happening after a few miles of driving, when things are nice and warmed up.

Now given that this is the same problem I was having last year with the blown head gasket, requiring the same driving style, this is my theory: At higher rpm the water pump starts pushing coolant into the combustion chamber, eventually killing the spark. I'm assuming at idle there's much less water pressure and the engine manages to burn off whatever's in there and is able to fire again.

The problem seems to be more rpm dependent than load dependent. Usually when it's an ignition system problem, the issue occurs with higher load, but I have no problems accelerating as long as I'm below about 3000rpm, and if I'm above 3000 it misfires whether I'm accelerating, decelerating, or cruising.

I'm hoping you guys might have seen an ignition component failure that behaves this way, cause I can't really see definite signs of a blown headgasket, but it's probably wishful thinking...

I get no white smoke, no coolant loss or overheating, can't see fluid in the cylinders or on the plugs. On the other hand my plugs are white (o2 logs are iffy at wot, sometimes .94v, sometimes 80's, but at cruise and idle they always cycle nicely), and the logs show low timing at wot, like 15 degrees, sometimes tapering, sometimes rising. The knock graph on the other hand is not any different between a steady cruise or revving in neutral versus a wot pull with low timing. I'm using ecu+ so all I get is a spiky raw knock voltage graph. I was runnig 50% 100 octane, and only 8psi on a 14b for breakin, so I have a hard time understanding how I could have blown a head gasket. (I was still a moron for not taking the time to look through my logs carefully, just relying on the knock graph). This is a stock composite gasket with copper spray and arp's torqued to 85ftlb with lube, retorqued after a few heat cycles. Both surfaces were machined. The only questionable thing is that I put it on a 40 over bore. It wasn't quite sticking into the bore, but it was very close. Hope I didn't burn the fire ring.

Oh the only other weird thing that bears mentioning is that I advanced my exhaust cam 5 degrees and retarded the intake 5 degrees to get my FFWD 272's to idle nice.

Anyways, that's a lot of conflicting info, that's kinda why I'm throwing it out there, in case someone's seen the same thing before. I really don't wanna take the head off unless I'm sure. (No, I don't have any vacuum leaks I know this is a newbie forum, but please no newbie replies )

 

[steel_3d]

I just experienced it today. Halfway through my trip I started getting the misfires, it was almost undrivable. Cleared the code right there while traveling on the highway, and it never came back for the rest of the trip, 25 miles. Weeeird.

 

[magnet8289]

I'm having the exact same problem in my car. It only happens when I'm driving on the highway, and I'm above 3k in 5th gear. Once I go from 4th to 5th, about 10-15 minutes later it will start misfiring and won't stop until I push in the clutch for a good 2-3 seconds and put it back in gear. Then it will happen every 15-25 minutes after that. It doesn't bother me to much because I rarely do highway driving, but I would rather it be fixed. I just have a stock 7 bolt, so maybe my problem is different than the guys running the 6 bolts. No boost leaks, and no other problems with idle, or just regular cruising. It only happens in high rpms in 5th gear after 10-15 minutes. Any ideas for me as I don't have a 6 bolt? Btw, I'm putting new plugs and wires in this weekend, but the plugs look fine, and the wires were changed not too long ago. I'll get a CEL when it happens, but it will go away by itself within an hour of regular driving. Any help would be appreciated.

 

[PieEyedPiper]

Sorry bud, but you're in the same situation as me.
Those who have had the 7 bolt misfire problem fixed theirs with a new CAS. I just replaced mine and it didnt help so I reset the ECU thinking maybe it is used to how the old sensor responded and needed to be reset to calibrate itself to the new CAS. I'm still testing that theory.
But ironically, I know of 2 other members including myself who only noticed this problem within a day or few of an aftermarket clutch job.

I'm still thinking a new black top CAS for steel_3d will help, that or DSMlink

 

[magnet8289]

Actually that is when it started happening to me too. After I installed my aftermarket clutch/flywheel, is when the misfiring on the highway started. I have no idea what it could be though?? It only does it at high rpms in 5th gear (which means I'm speeding haha), so it doesn't bother me too much, but I'd still rather have it fixed.

 

[PieEyedPiper]

See! There IS somehting strange going on fro us 7 bolt guys! I figured the clutch thing was merely a coincidence. I truly wonder what could be going on..

 

[magnet8289]

What would make it even more interesting is if we were all using the same type of clutch. I have a 6 puck stage 3, what do you and the other two people have?

 

[steel_3d]

I don't think the actual brand matters ACT 2600 street disc.

 

[steel_3d]

BTW I've been p0300-free for two and a half trips, 100 miles!

 

[4g64fiero]

I am having this exact same problem.

I have swapped in my 95 ecu with dsmlink and the problem goes away, ofcourse. Any 97-99 ECU I use will missfire once in steady condition above 2300 rpms regardless of gear.

Its seems as if, that the ecu sees the misfire and shuts off spark and fuel to the bank. My wideband goes completely lean during the missfire. I also have to let the motor go down to low RPMs before it recovers.

Really, this seems to be an issue with how the 97-99 computers are programmed. I am not sure its possible to get rid of the p0300 code for good without using the CPS that is intended for it?

I have a 1g green top on a 2g motor. The motor I just rebuilt around 5k ago.

I am just reviving this as every thread on this subject has went nowhere and I am having this issue right now. All sensors on my car are known to be good and working. I have tried 3 ecus and only the ecmlink ecu takes care of it. The only thing I am not sure about is the adjustment of the spark timing. I am not entirely sure if the cas is adjusted properly.

 

[steel_3d]

Holly back from the dead batman! Haven't been on this forum in years...

It's good that you brought it back. Not sure why I didn't update this thread (check my other threads), but if I remember correctly, the misfire that was left was due to ECU+ firmware bugs. Eventually the developer fixed the bugs and my car ran great, misfire free, until I sold it.

But a 1g cas will definitely NOT work misfire free on a 2g ecu without something like dsmlink or ecu+ to condition the signal.

One more correction to the thread! The blown headgasket on the old engine had NOTHING to do with the misfire I was experiencing. So my 1g cas NEVER ran correctly with the 2g ecu. I wrongly equated two distinct symptoms: the misfire and the blown head gasket.

As for the timing, I think I remember how I set it... There's no grounding wire on 2g's to set the timing, right? I think what I did was use the timing light and ECU+ real time logging, and try to match the numbers. So look at the range you see on the datalogger, then look at the timing mark with a timing light, and adjust the CAS until you see the same range with the timing light as the logs. Even though the timing is constantly changing, you can see the minimums and maximums it's hitting and match them pretty good. Then you know the timing the ECU thinks it's seeing is the actual timing.

 

[housegsx]

Each time this has happened to my cars, once on my 97 GSX and once on my 96 Tsi the culprit has been the ignition power transistor.

 

[apacolypse999]

i had the same problem as well did my friend. we both changed out or crank sensor and it went away no questions asked after that. seemed to good to b true, easy job done in 30 minutes

 

[19Eclipse90]

My '96 was enjoying the same problem after the 6-bolt swap. An EPROM ECU with a custom chip solved the problem for me. As far as I can tell since I didn't myself make it, the chip somehow keeps the ECU from checking for the random misfire. Only $50 for the chip from my local DSM/speed shop and the car has been problem free (in that area) since March.

 

[Black'95]

I'll revive this since I'm having the same problem after an aftermarket clutch install. It's an unsprung stage 4... Guess I'll try another ECU, and some other extra sensors I have lying around...

 

[myaceofdiamonds]

Any suggestions for me? I have a 6 bolt swap in my 95 tsi. Ecmlink v3 lite. My car started limping like a wounded dog while just cruising around. My laptop will no longer connect with my ecmlink and it all happened at once. Any suggestions??

 

[Black'95]

^^ No idea, bro... For the record, mine hasn't done it for quite a while, and I honestly can't remember what all I tried, but I know it hasn't done it since I did the valve seals and put in a new cam sensor. Cam sensor may have been the culprit...

I just remembered: I was also getting a CEL for MDP malfunction at the same time I'd get the P0300. I had an extra MDP sensor, so I took the old one off, cleaned the hole in the manifold where it mounts, cleaned my other MDP sensor, and installed. I can't recall having the issue after that, but from what I understand, the MDP sensor should have NO bearing whatsoever on how the engine runs. Weird.....