SteveW

Hi all, I have a 944 Turbo , I used to be here when I maintained it myself, though it's been looked after professionally for some years. Anyway, having spent a lot on it in the last year, yet again we had major problems last week, so I've decided to retire it into "weekend use" and buy a younger 968. I won't part with it, I can try to fix the thing myself as there is no time pressure on the job, I'm wondering where to start with this one, would appreciate any advice.

The problem started to show a couple of weeks ago, I was getting sudden temporary increases in revs, over 2000 rpm when you knocked the car out of gear (as was prudent!). The revs would drop back after 10-20secs to idle. I assumed throttle cable or linkage as the cause, and had not yet got round to checking this out. Then last week driving home the car started to misfire badly, backfires, black smoke behind, and was very down on power. The rev counter was fluctuating wildly, way faster than any changes in revs, and sweeping over most of the scale, never settling on a reading. We limped home just.

I wonder if the engine management has died, or whether this might be a sensor problem - perhaps a flywheel sensor failure could do this? I can't see why other failures would affect the rev counter display in this way. Would appreciate pointers as to where to start with this one!

Steve

mark944turbo

Check your current battery voltage as the guage flies around. If it is below ~10, low voltage is the cause. Probably a bad alternator not charging the battery. This happened to me a week ago, and shortly after the car died.

SteveW

Thanks, don't think the volts are low but I will check, from memory the voltmeter was OK, and no lights seen. Also the spouse started the car yesterday to move it a short way, 10 days after the event and lying idle, electric seats moving fine and normal cranking speed reported , so I doubt the volts were low. Interestingly it started well, though was still very rough.

Steve

bearone

don't bet the ranch on the voltmeter, use a decent one.

87951

Dal Heger

The first problem sounds like a dead or misadjusted throttle position sensor. Really easy to check. You should head a click from it when you just take the throttle off the idle stop. Most likely you won't hear it and that's where you're having the revs hang when you take your foot off the throttle. The DME won't "see" that you've come off throttle so it'll keep the engine revving higher than idle.

The second problem sounds like a bad connection from one of the flywheel sensors. Typically they will fail at the connections due to heat. Most of the time it'll cause the car to fail to start. Take a look at the connections at the back of the intake manifold. Most likely they'll be cracked/broken off.

Dal.

SteveW

Checked with my Fluke tonight. 12.47V pre start, consistent with 10 days idle and that one start followed by no charging much. After start rising to 13.7V and then dropping back as engine stopped, volts were consistent with slightly discharged battery under charge, I think the alternator is working fine.

Steve

SteveW

Interesting comments. While since I've stripped a 944 engine, ISTR the throttle position sensor is on the throttle butterfly spindle, will take a look when it's daylight :-). This car definitely starts well, absolutely as normal, done this twice now, it's a bit weird given the level of the misfire problem. I'll check out those sensor connections though, can't recall their location but guess I need to look at te top of the bellousing from what ou say.

Steve

VWaddict

Heh, Those Fluke numbers look spot on (I'm having to get a new 189 since somebody just tole my 87mkIII from work: With the upgrade, I'm actually quite excited!)

A high-resistance or high-impedance line from the position sensors would in theory lead to erratic tach position, (which is a symptom, right?) and would mess up the running of the engine since a sub-millisecond accuracy is needed for a true buttery-smooth idle.

I think Dag may well have hit the nail on the head, it makes most sense to me, and given that it's a frequently reported issue with these cars, I'd make that my very next point of inspection.

Look for cracks, crumbly/decayed insulator or corroded copper at the conductor ends.

Keith

mark944turbo

Well, it was worth checking. Good luck, ps I love my fluke too.

SteveW

Flukes are wonderful, just hate the continuity beep!!!

Steve

SteveW

Yea, think that's where to look next, going to have a go at this tomorrow. Found my old workshop manual set now, and it's actually really good on the DME. I'll inspect everything visually and get the meter on the relevant DME connector pins. I see that if I can get the oscilloscope into those two sensor circuits I can check the waveforms against Porsche specs if I need to go that far, It may be really useful to have some prints of the correct traces.

Steve

SteveW

OK, now inspected the sensor wiring at the back of the engine compartment, looks good. Then got the DME unit out, checked speed and position sensor resistances, both normal, no insulation fault on the speed sensor either. Next scoped the signals with the engine running, good sine type output on the speed sensor, normal amplitude, frequency varying with engine speed. Normal position sensor waveform also, as per Porsche manual. So they look OK.

Hmm, where to try next I wonder...

Steve

Dal Heger

Bad solder joints on the DME. When it's running well, hit the passenger floorboard with your fist (or if you have the DME accessible, tap it a few times) see if anything changes. If anything happens, then you most likely have a bad solder joint in the DME.

It could also happen when the DME gets hot, so try it when it's been running for a while. When it starts to act up again, tap it, see if it either dies, or fixes it. You'll know it's the DME.

Dal.

VWaddict

There's no (excessive) timing/sync jitter on the scope, I assume...

The oft-repeated "bad grounds" warning leaps to mind, but you sound clued-in enough to have had a good look at those as a first IA...

Keith

Ski

check pins 4 and 6 on the TPS and see if you have 1 ohm at the idle position. The raising revs is a classic sign of bad TPS or bad connection on the TPS. HTH.