S5 NA 3800-4000 RPM bog
 

Yes, I've searched. Sadly, lots of hits but no definitive troubleshooting guide in the FAQ. I did Kaboodle a bunch of threads, but again, nothing definitive which surprised me given that this seems such a common problem. If I've missed something (the search facility provides lots of noise but little signal) and there is a really good troubleshooting guide out there, I'd love a link.

From the threads, these seem the usual suspects:

1) OMP/MOP is f'd and killed the ECU just to make things more fun
2) Secondaries aren't working. Grounds or wires likely root cause
3) Aux Ports aren't opening. Seems to be an S4 problem with aftermarket exhausts and lack of back pressure. I didn't see anything about S5s having this problem.
4) Catalytic converter collapsed.

Background:

90 S5 NA vert. 130k miles. All stock but just did a a/t to turbo m/t conversion at Thanksgiving (dropping street ported T2 in at Spring Break). New fuel filter (along with ALL replaced fluids less than 18 months (and 8k miles) ago.

Car idles beautifully. As of Christmas has no "umph" (the technical term :rlaugh: ) from idle to 3800 but acceleration is smooth. Floored it takes a long time to go from 2000 to 3800. At 3800 it bogs noticeably but will continue accelerating to around 4200 but it's super pathetic at that point. Didn't push it past that. Not sure it would go past 4200.

The CEL comes on when you turn the key and never comes on once the engine is running. Assertion: that means the CEL is working.


1) Since the CEL is working, does that mean there are no codes?

2) Since the CEL is working, does that mean the MOP/OMP can be eliminated as the problem and hence the MOP/OMP has not assassinated the ECU?

3) Since the CEL is working, does that mean we are NOT in limp mode?


So I'm thinking it is probably 17 year old grounds (car has lived in AZ, CA, OR, TX) maliciously misleading the ECU; so I sent the kid to get dielectric grease and we're going to do Aaron Cake's procedure. I figure that's just goodness anyways.

The other likely culprit would seem to be the cat collapse since it is across the power band. But the sudden bogging at 3800 would seem to discount that possibility.

I wouldn't think it is the "typical" secondaries issue since the car drives like a Geo Metro (gutless) even from idle to 3800 when it would still be 100% on primaries.

Can we determine if it is a collapsed cat just by removing the exhaust and go for a ride? Unlike an S4 with the backpressure activated aux ports, since this is an S5 the only downside for a no-exhaust test is blood coming out of our burst eardrums, right?

thanks, appreciate any course corrections or other insights yall might have. We're in the 70's today and tomorrow so I'd like to get figured out this weekend.

thanks,

Jim

AFAIK you could take the exhaust off to check the cat. It's pretty easy to run a ground strap from the engine to the body, I'd try that too.

Update
 

Thanks, Letitsnow.

The first ground of the negative cable to the shock tower was very clean. There was no corrosion on the bolt at all and in fact it looked like it was a couple of days out of Home Depot. But we cleaned it all up with sandpaper and lathered it with dielectric. Now we've got the car up on jacks and are doing the starter end of the ground.

We took it out again for a ride (top down of course!) and nothing changed. We were able to take it up to 5k but it took a long time in 2nd.

We'll look in the exhaust once it cools and I think we'll pull the plugs just to be sure nothing weird going on there.

Yet another update
 

We cleaned the starter and ECU grounds. The both looked pretty good though. I wonder though if we should take the bolt that holds the ECU ground bracket to the UIM and clean it up too.

Next we have to do the one under the trailing coil (the one at the back of the engine compartment). In Aaron's write-up I'm guessing its an S4 because ours just has a bolt with a couple of ground wires instead of the copper block stuff Aaron talks about.

Pulled the plugs. they all look good. There is any indication they aren't firing.

Next step, pull the exhaust.

if it means anything my guts telling me ur primaries need to be cleaned. its a cheap service and they probably need to be cleaned anyways.

But so suddenly?
 

I'd think that would be something that would build up slowly instead of all of a sudden.

I actually have the T2 injectors from the engine we're building in a ziplock since I was going to send them to KGParts. It would be cool if I could stick them in and see but I bet they are too big. I'll have to research that.

Possibilities for the stumble:
Bad grounds
Bad TPS
Clogged Injectors

well you did mention that you had a new fuel filter installed 8k miles ago so it would seem very unlikely. my friend who had a 3000gt that started to bog really bad through out his whole rpm range had his injectors cleaned and then he said his car ran better than ever but then a few days later it was back to bogging. it was caused by a bad fuel filter. he grew frustrated and had his injectors cleaned once again and fuel filter changed. the symptoms never resurfaced again. i dont know, take it for whats its worth. if those t2 injectors are interchangeable i say go for it! couldnt hurt.

Injectors
 

They are bigger (sizes are listed in the FAQ). Who knows if they are clean enough to make a difference. May have to try em.

They'd make the car run too rich if their too large....

lax-rotor's suggestions sounds pretty good. have you checked ur tps? maybe one of them is stuck. check to see that they move with the throttle position but be careful if you push on one of them in too much like i just did to mine yesterday it could get stuck in. when that happened my engine didnt even want to rev past 3k. i was luckily somehow able to get it to pop back out with a lot of banging.lol

Tps
 

Yeah, The grounding is all done. But frankly it all looked good, clean and corrosion free.

He's pulling the cat-forward off now. If that doesn't seem to cure it we'll go ahead and check the TPS and AFM (anything else blatantly obvious?). I have the TPS from the T2 so we have that if necessary.

If that all checks out we'll pull the injectors and then send them off to get cleaned and we'll replace the fuel filter again. But damn it'll be annoying to have a high school senior without a car.:icon_no2:

I do hope someone confirms or denies my initial assertions that because the CEL is good that the OMP/MOP and ECU have been eliminated as suspects.

Looks like we're in "good" shape. It goes now with the cat-forward off (gotta wipe the blood off my ears!). Of course we changed two things including finishing the grounding and removing the cat-forward. We'll put the exhaust back on and see for sure.

Thanks for the help, guys.

You guys are forgetting that all of this happened right after a tranny swap. Would a tps go bad while yanking a trans? no. Would injectors suddenly clog when pulling a trans? no. Does a cat clog up when.....well....you get the point. Check the main ground wire first....the one that attaches using the upper left trans bolt. Double-check any connections that are in the area of the trans next.

Thx
 

We did all of Aaron Cake's grounding procedure. The main ground and tranny/starter ground (end of the negative battery terminal) did NOT make a difference.

Then we did the grounding procedure on the ECU ground, the dash ground (the one that screws into the back of the driver's side strut tower and under the trailing coil) and the tranny to firewall ground black wire on the passenger side). We also dropped the exhaust system at the same time. NOW, the car goes zoom, zoom, zoom.

So it is either clogged cats or it was that last three grounding fixes (none of which are really related to the tranny swap). We'll know tomorrow since we're going to put the exhaust back on. If it takes a dump, we'll know its the cat.

Frankly, I think it is the cat because there really wasn't any corrosion of note on any of the grounding terminals.

You're right, it could be a side effect of the tranny swap. But I'm betting that moving the exhaust system around as we uninstalled and installed it might have finally fractured what is a highly-stressed, rotary-abused (and in our case very old) catalytic converter.

We'll know tomorrow.

glad to hear it is most likely a clogged cat.

Mass air flow sensor plugged up or loose. Unlikey but it will cause you not to rev over 4000rpms. one more idea

It's the exhaust
 

We put the exhaust back on and it's so clogged up it would take a semi full of laxative to relieve the pressure!

Well, now we know, and knowing is half the battle!


I think we'll reenact the scene from Body Double and clear things up.

Hey just a heads-up. Don't just replace the Cat! If the cat overheated, then little bits of it are now lodged in the baffles of the exhaust. You need to replace the whole system, cat back.

Then, of course you need to determine why the cat failed. Was it old and just generally rotary baked or was the exhaust so excessively rich that the cat was prematurely destroyed? If the engine isn't running correctly, you can fry a new cat in a very very short time.

Make sure that the engine is tuned up in regard to ignition and induction-air filter, spark plugs, plug wires and O2 sensor. Additionally, injector cleaning would be a plus.

Good Luck!

Dont stop there man, replace that OMP ASAP! If it did indeed fry your ECU, then it could be tricking you! i.e. The OMP fryed my ECU, but only gave me a CEL for the water thermosensor..... fixed that, THEN the OMP CEL showed up (thank god i premix).... fixed that, THEN the Knock sensor CEL came up. Get the idea? Fix it before it kills your car!

The backside of the cat looks good so the collapse has all been contained in there. I am rebuilding a Tii to swap in at spring break so I dont want to spend any money on na specific stuff if I dont have to.

We know the CEL works and we have no codes so there is no reason to believe the OMP/MOP is bad. We do know the cat section of the exhaust is causing the problem.

Hollow it out if you don't want to spend the $ or, if you want to go a more legal way, get a hi-flow.

to me it sounds like a tps is not hooked up properly but make sure everything is connected right that my opinion.

uhh:uhh: vrracing has already said it was his cat, no further input is required