Major malfunction & I can't find it.

[MeanBuicks]

Geeze! I've been around these goofy cars long enough to be able to pinpoint about anything but the old GN has got me flamboozled!

Under a load it runs real clean until 4000 rpm and just hits a wall. The engine just breaks up and stutters real quickly just like the stutter box on a bracket racing car. Won't do it at all below 4000 rpm or if there is no load. In park/neutral, it'll wing right up to 5000 rpm quicky & cleanly. Here's the skinny:

- New Comp roller cam, beehive springs, pushrods, double roller timing chain. (About 3000-4000 good running miles on this stuff.)

- I've checked the turbo, pulled the intercooler & cleaned it.

- Tap tested the MAF and monitored it with the Scanmaster. Seems OK.

- Checked all the grounds under the hood.

- I've swapped the coil & ignition module.

- swapped ECM & chip

- swapped ESC module & tried unplugging the knock sensor

- checked plugs & wires

- readjusted cam sensor

None of this has changed anything one iota.

Is it possible that a valve spring/springs have gone sour? Would the engine run like a top with no load if there was a valvetrain problem?

Help?

 

[st1nger]

shutter

did you check cat for blockage or could it be a tranny problem does problem occur just reving engine pass 4000 rpm or is it under load just a suggestion

 

[MeanBuicks]

Good suggestions. There is no cat on the car (or mufflers) and the transmission is fresh. Not sure what could be wrong with a transmission to make a car act like this and I work with them everyday.

 

[III'S 87 GN]

what harmonic balancer do you have ?

see if it is loose !!!

 

[Russ Merritt]

Similar thing happened to me once but it wasn't limited by RPM it was limited by boost. Anything over 10# and it would knock and miss somethin aweful. After 23+ hours of checkin everything it turned out to be a broken wire on the crank sensor. Car ran flawless till I got on it then all hell would break loose.

 

[Scott Atk]

Had a similar problem on a local car recently. Ended up being the schrader valve from the fuel rail lodged inside the fuel pressure regulator.

 

[MeanBuicks]

Original, stock balancer. I'll get under there and check it and the crank sensor wires. Could be onto something there as it certainly seems ignition system related like it's getting confused for some reason.

The car has 233,000 miles so I would expect anything to go awry. Thanks for the suggestions guys!

 

[ijames]

Any chance you can get enough rpm/load by brake torquing with the car sitting still? If so, do that with a timing light and you will check the crank and cam sensors. How about fuel filter? How many miles on that?

 

[bruce]

Try a new TPS.

 

[Doug Jacobson]

I chased a very similar problem to this on my car recently. I'd suggest knocking the plug gap down to .025 or so and see what happens. It cured it for me, maybe it will work for you.

 

[MeanBuicks]

I can't seem to induce the problem while brake torquing the car. I changed the fuel filter just this afternoon but forgot to put that in my initial post of things I've done.

I suppose I could get another TPS on there & try that. It seemed to be reading correctly according to the Scanmaster. Car starts right up & idles correctly too.

If going to a smaller plug gap helped, wouldn't that be indicative of a weak coil and/or marginal wires?

If the weather is cooperative tomorrow, I'll delve into these ideas. Thanks folks! This crap can get frustrating.

 

[Under Pressure]

Originally posted by MeanBuicks
I can't seem to induce the problem while brake torquing the car. I changed the fuel filter just this afternoon but forgot to put that in my initial post of things I've done.

I suppose I could get another TPS on there & try that. It seemed to be reading correctly according to the Scanmaster. Car starts right up & idles correctly too.

If going to a smaller plug gap helped, wouldn't that be indicative of a weak coil and/or marginal wires?

If the weather is cooperative tomorrow, I'll delve into these ideas. Thanks folks! This crap can get frustrating

I had the same problem it was the plugs they where brand new (r43ts ac delco) the problem had me stump for a month ,till my friend had a problem with a car at PTE when he worked thier. They put in a AC delco CR43ts. Problem solve i tried it and it also worked to solve my issues. gapped @.032

 

[Doug Jacobson]

Originally posted by MeanBuicks
If going to a smaller plug gap helped, wouldn't that be indicative of a weak coil and/or marginal wires?

Possibly, I put anew coilpak and wires on at the same time too, but since you mentioned that these were new I didn't push the issue. But I would OHM your plug wires, my old ones had some resistance but is was acceptable (770 milli ohms or so) The new ones tripped my meter over to full continueity. I had a race coming up and just figured I'd try all at once to try and eliminate the problem. It did and ran .3 faster than before.

DJ

 

[gnsrule]

I ran into a similar problem on the 86. I thought it was a coil/module combo, swaped on a new one from Napa, still the same crap. In the heat of it all, I swaped on still another new coil/module combo this time from Autozone. Still fricked up. As a last ditch effort, I got a GM module from John Spina. BINGO
If your not useing GM modules, Id Highly recomend getting one even if it doesnt cure your problem. I found out the hard way.
Good Luck

 

[bruce]

Originally posted by MeanBuicks

I suppose I could get another TPS on there & try that. It seemed to be reading correctly according to the Scanmaster. Car starts right up & idles correctly too.

The sampling rate of ALL scanners, and DS are way too slow to catch some TPS failures. You need a good scope to see the havic on the signal line, from some of the failures. I chased the same thing around on my car for months, and just as an act of desperation did the TPS, and it cured the problem. Reinstalled it and looked at it with a scope, and went, dammmmm.

 

[MaxVO2]

If you think about how much use the TPS gets, and how important it is, I just keep a spare around for when this sort of problem comes about, along with a crank sensor, coil, ign module, MAF, etc... Heck, I should just buy a whole other GN just so I can swap parts between them to diagnose problems. I've spent enough on them!

I have a friend whose an electrical engineer, and he scoped my apparently bad TPS, and used another tool to test sampling rate on various types of EGT type probes I had, accuracy, etc..

Compared to what my Scanmaster was picking up for the TPS, A LOT of info was missed, even moving the throttle slowly, etc.. Cheaper to just have an extra on hand. I always wonder how much the typical scan tools we use miss in terms of knock, etc.. I guess they must work well enough for our applications since people without these tools sure seem to post a lot of messages regarding head gasket preferences and rebuilding specs on 14 second, mostly stock builds. For what it is, and what it does, the Scanmaster is an awesome value. The scope my friend was using is a $5k machine! That will buy a lot of TPS's, coilpacks, and genuine GM modules!

Billy

 

[bruce]

Originally posted by MaxVO2

Compared to what my Scanmaster was picking up for the TPS, A LOT of info was missed, even moving the throttle slowly, etc.. Cheaper to just have an extra on hand. I always wonder how much the typical scan tools we use miss in terms of knock, etc..

Any scanner that uses the ALDL data link is limited to the 160 Baud rate of the GM ecm. DS reads the RAM locations, as their in use, and can sample things much faster, but even with ~15 frames a second, it's painfully slow compared to watching things on a decent scope. Not to mention that some of values stored in RAM have been filtered thur the ecm code.

Luckily the ecm is processing information, quicker then we can extract it, with the tools we have.

 

[BJM]

Try Ohming those plugs. I have had more than one bad AC plug. Measure the center electrode to where the wire connects. I always buy 8 plugs at a time and measure them all.

 

[ACME RACING]

...hoses

Mine always does this when I mess with the intercooler hoses. They looked good and tight, but one hose was wasn't sealed good and blowing out boost when boost got above 10lbs. I realigned the hose and clamps,tighting them and no more cut out ..... Just my .03 cents

 

[83ttypecooled]

I agree with Bruce that a lot of TPS failures can't be seen properly with out high speed sweep . On my snapon vantage, occasionally I can see these sudden open circuits in the TPS, but just as often will miss them. Some time you need to get into the micro second range to really see it.However, I think it is just as likely you are dealing with an ignition failure; the system is doubly important on a turbo car, has to be in top form.