'85 carrera power loss

[TZ 197]

Was hoping to get some input on my problem. I have a '85 euro carrera and after a slight over rev trying to get merged on the freeway the rev limiter kicked in and ever since the car is very lazy. Had a compression test done and 175 in all cyls. so my bent valve theory is out? Maybe I need to do a leak down to confirm no valve issues. I also had a full tune and service done at that time that made the car run smoother, but no power gain.

It's difficult to quantify to my mechanic the performance loss as this was the first trip to his shop but it really is weak now by comparison. His shop is well known and he only works on Porsche's but so far no luck getting it to accelerate better. I was hoping maybe the fuel cut off system has a weak link that might be the problem or something silly and common after such an event. The car really isn't much fun to drive any longer.

Any help is greatly appreciated.

[CruiseControl]

"Very Lazy" would indicate significant power loss.
Did the "Full Tune" include rocker adjustment. If not, have you checked for broken exhaust rocker(s) ? An exhaust valve not opening will robb power but still give a reasonable compression test figure. Also check inlet rockers.
Should there be a failed rocker it may be a result of a "stiff" (or is that stuffed) valve.

Paul

[wolf]

if the revlimiter kicks in and the engine is warm i would not expect catastrophic damage...

[TZ 197]

Yes the work included a valve adjust and all were very close to tolerance. My last adjustment on valves was less than 2,000 mi. ago. Something makes me feel it is in semi cutoff mode causing lack of fuel pressure. Isn't rule of thumb more pressure, more fuel = HP. Can the cut off solenoid get stuck?

[Rot 911]

If you redlined during upshifts I don't think the redlining was the cause of whatever the problem is. I bump off my rev limiter all the time.

[TZ 197]

The issue started immediately after my upshift over rev. The lack of mid range grunt was instantly noticeable.

[dentist90]

Originally Posted by TZ 197

. Can the cut off solenoid get stuck?

I'm by no means an expert on the electrical stuff, and we all know that the Motronic limits revs by cutting off fuel.
I was under the impression that the DME just shuts off the injectors at 6250 +/-.
I don't think there is anything to get stuck, per se. Steve W or Ingo Schmitz or one of the other electronic gurus may have a notion as to what may be wrong.

While you're waiting it may make sense to check the fuel pressure at the injector rails to make sure the fuel pump check valve isn't stuck.
I've hit my rev limiter several times with no ill efects.

[TZ 197]

Is the check valve the point of restriction when the electronics shut off fuel flow to limit rpm?

Also what should the fuel pressure be under normal circumstances? TIA!

[TZ 197]

Anyone? Please. My baby is sick and so am I.

[dentist90]

The check valve is at the fuel pump. As far as i know, the rev limiter acts by stopping pulses to the fuel injectors and is controlled by the DME module. This will allow immediate fuel cutoff and return of fuel when revs are back in range. If I am wrong (very possible) and the DME works to stop the fuel pump there may be something that has caused the fuel pump to not deliver enough fuel now, but I don't know how or why hitting the rev limiter would cause the problem you are describing.

Anyway, the residual fuel pressure at the rail is 36psi (engine off) and about 30psi with the engine running according to the Bentley manual. The test port is on the left fuel rail above cylinder #1.

[dentist90]

Just wondering, is the problem just sluggish acceleration, or will it not reach highway speeds at all? Does it rev thru the full rpm range OK? Does it hesitate or buck, or is it just slow and smooth?
Maybe (?) that was not your rev limiter kicking in but a sudden failure of some other part, such as the air flow meter or fuel pressure regulator. I think we may need a fuller description of the problem you are experiencing.

[jimbauman]

TZ - sent u a PM....

[ischmitz]

dentist90 is correct - the DME just stops sending fuel injector pulses when the rev limit is reached. Nothing else is going on and nothing is "stored" in the DME. The electronics do not suffer or get stressed when you hit the rev limit. Starting with the 996/986 the DME "remembers" how many times you hit the rev limiter. This can tell whether you drove it like you stole it when it comes to warranty claims.....

I don't see a direct reason why your car would have lost power only by hitting the rev limit. And since your compression checks out fine I'd pretty much rule out mechanical damage to the engine.

How different is the feel of the car. I mean is it very obvious or are you just worried and maybe exaggerate?

Ingo

[TZ 197]

The car runs normally other than it no longer has any decent pull and revs slower. It feels like it lost 40 hp. Other than that it runs smooth, idles fine and is capable of revving through all gears, albeit slower. No stumbling or hesitation to speak of.

Seeing as the problem arose with the fuel pump cutting off I have to think it is in the fuel delivery circuit. Fuel pressure tests are my next step it seems. Am I correct in the thought the 3.2 has a low and high pressure pump as with other motronics systems?

I guess it's time to buy a manual. Thanks to all for their input as I'm not going to sleep well till this is resolved.

[TZ 197]

Ingo, I wish this was just my imagination but there is a big difference from before. The car used to pick up rpm's quickly and be torquey, now it is flat and linear. Dante

[dentist90]

The fuel pump is capable of high pressures (>75psi). The pressure regulator limits it to ~36psi and dumps the rest back to the fuel tank. So the pressure at the injectors is about 30-36 psi and the flow of fuel into the intakes is controlled by the pulses of the injectors.

Definitely get yourself the Bentley manual and check out fuel pressures to start. There are several things to check after that: spark plugs and ignition wires, fuel injectors for instance. Perhaps you just have one dead cylinder due to a bad plug or wire.

[TZ 197]

Wires and pressure test are def next. Had new plugs installed during recent service. Will also check injectors as maybe the fuel surge broke some crap in the lines loose and is plugging one or more of them.

A previous lack of being able to fix an issue and tons of money thrown at it led me to sell my '87 turbo and this just feels like Deja Vu to me. It also made me look for another mechanic this time around.

Had I known about Pelican with the knowledge and tenacity of it's followers back then I might still have that car. I have missed it every day since and won't give up this time.

Many thanks again to all who have helped out!

[ischmitz]

A couple of simple tests: Let the car idle and then disconnect one spark plug wire at a time. RPM should change every time slightly. Then do the same with the injectors. Again RPM needs to change every time. Maybe you'll find a dead cylinder.

Next would be to make sure your fuel filter is not old/clogged. A fuel pressure gauge might be a good idea at this point, too. Unfortunately, you need to monitor fuel pressure while under load. Simple reving doesn't do it. So maybe a long extension so you can take it into the cabin while an assistant drives. This could help narrow it down to fuel starvation.

Keep us posted,
Ingo
Ingo

[TZ 197]

New fuel filter in service also. I now have some ideas to work with and will keep you updated.

[Rot 911]

It may be fuel pressure, but I think it is probably something much more simple. Check to see if maybe you popped off a spark plug wire. Make sure they are seated both on the plug and the distributor. Make sure your distributor isn't loose and shifted. Check your air filter, see if it got sucked in the afm (unlikely).