Need help! I have no idea where to start. . .
 

The last few days i've started the car in the moringing its been acting up. It revs up to 1200 rpm for a second then drops to just over a thousand. By the time i get down the steet a few hundred feet its at 800 rpm or so and rough, rpm kinda of fluctuating up and down rapidly and inconsistantly - almost like it was starting to loose fuel kinda feeling. Once warmed up the car seemed to run fine, but somthing still was off and I wasn't able to put my finger on it. It seemed a litte underpowered. Also, the car use to start right up. Now it starts kinda like a train starting. Meaning it kinda builds revs up.

Today, same scenario, except when I get in my car after work and start the car the rpm bounce up and down between 1800 rpm and 1000 rpm crazy like, for like 10 seconds. I had to give it gas to sort of settle it out. But as soon as i started pulling out the car began bucking, like it was starved for fuel and then all of a sudden the fuel cut in, or like the ignition was cutting out and back in again. This would happen inbetween each gear change just as I began to give it gas. The only thing I could do was rev it between changes to stop it from bucking. Once on the road, I would try to acclerate, I would push the gas and it would accerate for a split second then nothing, the car would just coast for a second or two, then all of a sudden it would go again. It would cruise fine once going but the craziest thing was the Tach. It was fine at idle, but as soon as i gave it gas the rpm read about double what it should - even cruising. Second gear at about 20 mph read at over 4000 rpm. So i babied it to the hair salon where i had my appointment. 45 min. later i go outside and the car is back to what it was several days ago.

I have no idea. It feel like an ignition problem. I would have thougt fuel filter, but the rpm thing makes me thing otherwise.

Any help guys:(

1. Check all your vacuum lines behind the throttle body and to the fuel pressure regulator (left side fuel rail) and fuel pressure damper (right side fuel rail).

2. Check to make sure you have no vacuum leaks around the 90 degree elbow that interfaces your air flow meter to your throttle body. Also check and tighten your clamps which joins the two intake plenums together.

3. Remove and inspect/clean out your idle control valve.

4. Inspect your spark plugs, distributor cap and rotor and replace if required.

5. Check your fuel pressure. Should be at around 36-38psi at idle. If it's fluctuating a lot, you may have a blocked fuel filter or a stuffed regulator or damper.

6. OH, stick your head near the throttle and check to see if you can hear the idle position microswitch engage when you release the throttle (engine off of course).

Just a few things to get you started.

Re: Need help! I have no idea where to start. . .
 

Man, I dunno but I think this is the problem!!


:D

Re: Re: Need help! I have no idea where to start. . .
 

If you saw the girl that cut my hair, you might be tempted to make and appointment:D

1. Vacuum lines are ok
2. same as 1.
3. Will, do, but i can hear it humming.
4.just replaced all wires and plugs a 100 miles or so. Check rotor and cap at that time too.
5. I do need to do this. Will do.
6. I have and I do.

I don't think its mechanical in nature. Like I said the engine ran fine after i stopped the car and started it again. But the thing that make me think its electrical is the tach and the way it was acting. It seems if it was mechanical in nature the problem would be so intermittant. However I could be wrong. Would a bad DME be a suspect here?

CHT sensor?

might anybody else have any info they can share?

Well, there's always the reviled DME relay...

ianc

yeah, just ordered one - even if its not the problem, its good to have one. From the reading ive been doing it seems that the DME thing is an all or nothing thing, meaning your car runs or dont'.

The thing I cant figure out is what would make the tach act so crazy? If it wasn't for that bit of info I would think its a fuel issue or ignition issue.

From the systems described could be many things

But I would hazard a guess that these might be at fault
1) faulty DME
2) flywheel sensors
3) Mass airflow meter

Michael

i vote for maf....if u r handy with a volt meter ... maybe someone can post link for maf diagnosing.

Gonna have to go with Michael on this one. My guess would be the DME or the MAF Sensor. Click here to see why.

Randy

Actually, from what I've read, the DME relay can indeed be an intermittent problem. You'll soon find out however, if you've ordered another.

Just as a point of reference, the standard 'barn door' air box on the 84-88 Carreras is not a 'mass air flow' sensor. It has a flapper valve inside which measures air volume. There is a built-in temp sensor, which could be faulty (unlikely), and is not replaceable (new airbox). These two measurements, taken together, allow a calculation of the air's mass, but it is not measured directly like the later 'hot-wire' MAF systems.

Randy, from what you've posted before, I believe you have one of these fitted as an upgrade.

rcaradimos posted to check the Cylinder Head Temp sensor. The original design has two wires and depends on grounding through the head, which is unreliable. A faulty CHT sensor will give the wrong mixture and make the car run like crap throughout the range if it malfunctions.

My sense, from an attempted distillation of your problems, is that your car is having trouble dealing with throttle. It bucks and cuts in and out when you try to accelerate, but runs OK at steady state. Start times are of course affected.

My first guess at this is a vacuum leak. You won't be metering fuel for the extra air, and if the throttle switch is closed, the DME will try to compensate for low RPM in various ways.

Check the vacuum connections to the pressure regulator and dampener on the front of the fuel rails. Granted a bad connection to the regulator would give worse performance at idle than open throttle, check it anyway, and at its other end where it joins the throttle body. There is a 'Y' connection there which can be flakey.

ianc

Thanks everybody for your comments.

Ianc: I will certainly look into these vacuum lines. But I'd like to follow up with a few comments / thoughts I had last night thinking about this. It seems what is happening is that the car is either getting zero fuel to all 6 cylinders or the spark is cutting out completely by the way it feels. I'll push the pedal and its like the car died, then all of a sudden, bam, acceleration. Maybe this is a MAF failure, but if it was wouldn't it be "broke" all the time? Remember, when I got back in my car it ran fine. A question I have about fuel pressure on a fuel injected car is if you have pressure loss would the car just completely cut our or would it act like its struggling to run?

Would a bad coil be a culprit to concider here?

I don't know how my tach issue feeds into this. Somehow i feel its connected since it did what it did while the car was running bad. What sensor gives the tach its actual signal to work?

Thanks again for all the help

I suppose it's possible. The crazy behaviour of the tach makes me think it's ignition related rather than fuel, since I believe (not sure of this, I'd have to check the Bentley), that the tach gets its signal from the ECU. How are the grounds on the LH intake manifold looking? Have you done a visual survey of electrical connections throughout the engine compartment and elsewhere?

ianc

I've only had a brief look since this started yesterday. However, the grounds (and im assuming the one you can see on the intake runners on the left) seem solid enough. I'm going to start going though it tonight. It just such a vague thing to me Im not sure where to start. Im guessing I'll start with the easy to see and test stuff first like vacuum and wiring. It definately seems ignition oriented to me too.

Do you think that this could be an ECU thing? And how do you test it?

Similar situation occurred to me a while ago, after putting "bad" fuel in the middle of nowhere... - no choice. The fuel distributor gummed up and the plunger did not float as it should creating a weird uncontrolable low RPM's issue. Took it apart, cleaned it and bingo the problem went away.

Yes, I suppose it's certainly possible, although from what I understand, outright failures of the ECU are far less common than electrical connection or component failure issues.

By swapping in a known good one? ;)

ianc

Id take it to your local P-car Mech.


I went exactly through what you went through. problem was intermittant at best. THis is what I did.

1. Bad fuel, no
2. bad coil, no
3. bad vac, no
4. bad dme regulator, no

Took to mechanic, and he fount to be MAF like barn door thing. He stated they wear out and and when they start to open , the go OPENLOOP causing a lean condition and then you cant get rpms up.
This is just my problem and might not be yours.
So, if ya try 1 - 4 and it dont work, you can always get a new Barn door Maf. Not cheap.

There in lies the rub. There are no good P-car mechanics where I live, the nearest would be about 2 hours away. We have a shop that works on European cars, but they do not specialize in them.

Could a bad "barn door" MAF be intermittant?

I guess Im in for one hell of a learning experience with this electrical stuff cause I just don't get electrical at all. It seem strait forward enough to test stuff. I'll need to go out and get a decent digital meter and just give the ole go.

Ianc: you said the DME could be intermittant, did you mean the car can be running while the DME is having a problem, or did you mean it works one day and the car is fine and the next day not at all. In otherwords, can a bad DME cause the car to have iginition issues as i've desribed but not shut the car down totally?

It sounds like you need to begin educating yourself on electricity & electronics - that's the long term project... get something simple from the city library or a community college. Then go to the library at USL (or whatever they're calling it these days). In parallet, find the Bosch booklets on the theory of operation of the stuff on your car and try to read thru them.

Sorry I don't have an easy solution - but not having a shop nearby means you are a DIYer, even if you don't want to be.

No, it can be while the car is running. You may be getting intermittent contact in the relay due to heat, moisture, or other issues that will give strange symptoms. I don't get the tie in here with the tach though...

ianc

I just PM'd you greg

How come no one questions the state of the CHT sensor like rcaradimos suggested?

I appreciate everybodies input - really, I do.SmileWavy

Ianc, hate to keep bugging you, but you mentioned the tach got its signal from the ECU, but doesn't the ECU pick it up from somwhere else. Does the flywheel sensor contribute to this.

I knew somthing was going on with this car a few days before the went wacko with the rough start and idle.

I know there is some sort of check valve that holds the fuel pressure, like i previously mentioned in the few times i started the car in the morning (the last few times) Its started rough and had to build up speed - almost like a locomotive before it got smooth. This led me - at the time - to believe it was fuel related - do any of the previous suggestions metioned by the various posters help key in on what this might be - I think its a clue to follow, but Im not sure what to check if it isn't the check valve bit.

Sorry for so many questions, but I'd rather be thourogh than sorry, again thank for the advise.

Yes, there is a speed sensor and a reference sensor. The reference sensor, IIRC, is only used when the car first starts up. The speed sensor gives the ECU RPM information, so logically it would send that to the tach. Both of these are mounted under the car on the driver's side near (obviously) the flywheel. Their electrical pickups are in the engine room on the driver's side in a small bracket to the left of the intake manifold.

ianc

Edited to say: if you don't already have the Bentley Carrera manual, I would jump on it ASAP...

Your symptoms didn't sound exactly like mine but it could be a possibility.

Try this link for info.

http://forums.pelicanparts.com/porsche-911-technical-forum/273451-finally-problem-solved-hesitation-etc.html

I have the Bentley and the Haynes, and im trying to understand the interconectivity of the whole system, but it helps to hear others input in conjunction with the books.

When my CHT sensor started to fail my rpm's were jumping from 2000 to 400 to 1800 to 800 etc. start no start etc. As JW puts it, rip the old one out and throw a new 2 wire in. I'll see if i can find the how to post.
Good luck

Lot of good info in this thread...
to answer one of your questions...

""Could a bad "barn door" MAF be intermittant?""

Yes, it was intermittant , but progessily worsened.

1 time a week became 1 time a day, became "Honey, follow me to the Mech shop...I hope it make it."

Here is an update:

1. the CHT is of the newer type, and looks to be rather new. Not much grime or dust on it compared to other compents in the car.

2. The flywheel sensors seemed pretty good to. The black rubber hosing isnt cracked and very fexible. I removed them and the only thing i noticed was that there was a small metal pile of filings (presumably from the clutch) magnitically attached the end of the senor. Could this cause a signal pickup issue?

3. the fuel pressure dampers vacuum line as barly in the flexible hose that attaches to the damper.

4. I tried to test fuel pressure, but my fuel pressure tester from autozone doesn't fit. Does anybody know where i might find one that fits? An observation that may or may not have any relevence. When I removed the cap to test fuel pressure the gas just dribbled out - didn't squirt out like somthing under pressure. I know there is a fuel pump check valve that is suppose to hold pressure in the system for start up. Remember, i have had problems with rough startups.


While resistance tests can only prove if a compent is broke and not 100 percent working here are the results:


IAT: 1.9K OHMS AT 80 DEGREES F
CHT: 2.01K OHMS
FULL THROTTLE SWITCH: .2 OHMS
IDLE POSITION SWITCH (42 AND 44): .2 OHMS
IDLE SPEED CONTROL VALVE: OUTTER TERMS: 42 OHMS, B/T CENTER TREM. AND EACH OUTER TERM: 21 OHMS