Help me diagnose - poor idle/rough running/low power from engine
 

WIN FREE PARTS FOR CORRECT DIAGNOSIS!!!

This one is a bit of a headscratcher. I'll even offer an incentive - the first responder to this thread to diagnose it correctly gets some free 944 parts - their choice of a set of fog lights, tail light lenses or a new cam tower cover. Maybe something else of similar value (we can negotiate) - point is, you'll get some free parts. . .

If you think more information is needed, I can answer "have you checked this?" questions - just post 'em below.


- - -


Car is an 85/1 944.

Here are the symptoms:

- Car starts right up normally & quickly, vroom. No problems there.
- Idle is a little bit rough (kind of like maybe one cylinder is intermittent?)
- Fuel efficiency/MPG gauge (which is normally all the way to the right at idle) occasionally "twitches" to the left in conjunction with the rough idle/misfires.
- When you put your foot on the accelerator all the way down (WOT/floor it), the engine will bog down and "pop" loudly once or twice or possibly stall out completely. In any case, it feels like it isn't good and probably not a great idea to continue doing.
- If you put your foot about 1/3-1/2 way down, the RPMs will come up, but very slowly/gradually. The engine doesn't quite sound right - more like a Honda lawnmower than a Porsche.
- The tach gauge appears to register correctly at idle, but as soon as RPM is increased it seems to be off by about 50%. If my ear tells me it's at 3,000 RPM, the tach will indicate maybe 2,000 RPM, for example.
- Extremely difficult to drive due to lack of power/engine not wanting to "spool up" & roughness/misfiring occasionally
- A small puddle of blackish crud (oil?) underneath the tailpipe outlet when I started it and got the RPMs up to (what my ear tells me was) about 4,000 RPM & held it there for about 30 seconds. Could this be simple oil fouling?

Here's what I've checked so far:

- Pulled the DME & checked the internals. Everything looks good visually. No burned smells or obviously failed solder joints.
- Charged & checked the battery
- Checked distributor cap/rotor
- Checked plug wires
- Confirmed spark at each plug
- Confirmed fuel pressure at the front of the rail (flows when key is "on")
- Checked coil output
- Put some Seafoam in the fuel to loosen up any crud
- Checked all the vacuum lines for proper seating/no leaks
- Checked connections at TPS/AFM



What else should I check? This one has me kinda' stumped. Car was running fine, then went out to start it one morning and it ran like crap (as described above). It's been this way since with no improvement.

Jeff, check the reference sensors, it may have come loose

. . .and their plugs.

Well I have been posting this a couple of times this week alone...
TPS
http://www.clarks-garage.com/shop-manual/fuel-06.htm

But I will continue to think about it.:cool:

John_AZ
1988 924S + 1987 924S

I had a similar issue about 6 months ago with my 85/1. After much swearing and head-scratching, I found out that the injector on my #2 cyl was causing the problem. When I took it out to find out what was going on, there was a chunk of crud in the injector that was causing fuel blockage occasionally.

It was an easy fix, and haven't had a problem since!
Hope it helps!

-Nick

My first guess is the auxiliary air valve. It regulates extra airflow at cold idle to match fuel delivery. If it's stuck shut or even partly shut you'll get a really rich engine on cold startup - it'll do what you're describing. Then you'll get too lean when the motor's warm.

Also, I'll second many944's injector call.

Again with the double post! Sorry.

your timing is off for some reason. this explains the odd noise when accelerating, the odd tach readings, the crud, the popping, the stumbling, etc. i am going to second the issues with the speed reference senders, et al. could be a bad ground, maybe even a bad computer.

I don't think this is it but I would confirm that the barn door is working (not jammed) and the slide contact is good.

Thanks for the responses - I'm planning on digging into this a little this coming weekend.

I'll check the TPS although I'm inclined to doubt it's that simply because of the weird erroneous tach readings issue. Don't think a faulty TPS would cause that (but it might). Similarly it's difficult to see why the tach would be reading low with a bad/plugged injector but I'll check proper flow on each one.

Simply because of the tach issue I'm inclined to think this is either a speed/reference sensor issue (although oddly the car does start/run okay, I'd expect to have difficulty starting if these were bad).

Should I be looking at oscilloscopes? I've never done the speed/ref sensors and my understanding is that in order to replace them you have to drop the transmission out of the car. God almighty I'm hoping that's not the case. Anyone know for sure? I think I can diagnose them in the car with an o-scope, but assuming they're bad, how much of a nightmare am I looking at?

i am thinking that it may be a spacing issue with the reference sensors. it is close enough that the car starts fine, but the signal gets weak when the flywheel is spinning.

How would that spontaneously "just happen" though? The car was running fine and then one day "poof" out of the blue it just started running like crap. I'll check on Clarks for a diagnostic procedure but I dunno if I can adjust those things without it becoming a humongous ordeal.

Breathe a sigh of relief....
You don't have to drop anything to swap out the sensors (though you will drop what ever tool you use to remove those d@mn allen screws that hold the sensors in about 20 times in the process :) )

If you have a copy of the shop manual, it shows how to glue a washer to the bottom of a non-functioning sensor to properly set the spacing of the holder assemb. If you test both of your sensors and they are good but you still want to change/ check your spacing, I have an old sensor already modified for this purpose. I could drop it in a padded mailer and have it to you in a couple of days if you need to borrow it.

let me know!
-Nick

just for grins, pull the plugs on the sensors, spray them with starter fluid, apply dielectric grease and see what happens.

Thanks guys - I just checked Clarks and the procedure actually looks kind of simple. I'll try it out and report back.

And blood. . .lots of blood.

Pleeease don't put dielectric grease on the contacts. It's an insulating substance not a conductor.

But don't take my word for it, read for yourselves;

http://en.wikipedia.org/wiki/Dielectric

I know folks have used it "for years" without any problems but that is because if you use it lightly when the contacts engage they tend to scrape the grease off. If they are a bit loose and you get enough of this grease in there it will tend to increase the resistance.

It actually works very well in spark plug rubber boots and others as it keeps the boots lubricated (does not break down due to electrical charges) and prevents voltage leaks.

I would try to borrow a good used dme as it sounds like a computer problem or wiring to computer to me.

I had a similar problem when I bought the 951. The BOV was shot. I replaced it...and it didn't fix anything.

Come to find that one of my vacuum "T" connectors had a crack in it.

Dang it! I was way slow in this post. I went w/ ref sensors too. Also maybe a bent AFM flap, but that would have entailed an initial backfire caused by something else.

that is correct. that is what the word 'dielectric' means. which is also why i put it on my connectors. it prevents electricity from going where it shouldn't and it keeps water and corrosion out.

http://www.jamestowndistributors.com/userportal/show_product.do?pid=8196

Hey Jeff,
Just seing if you had resolved this issue yet?

Hi, nope. The poor 944 has been sitting since I asked these questions and I haven't had time to look at it. I am however planning on getting into it this coming weekend (I actually have some time now). First I need to get my motorcycle out of my garage which is in the final stages of a LOT of upgrades and then I'm going to roll the 944 in and get this thing fixed. Once that's done I can get back to focusing on my 911... *sigh* too many projects...

Anyway I'm going to follow the advice here and check the injector flow rates first, spark secondly at each plug and then if those come up good, focus on the sensors.

I miss my 944 and want it back so I can enjoy it again!

Wouldn't hurt to check the plug at the AFM - mine was gunked up and causing trouble. Easy fix to clean it up. GL & let us know.

I myself am having some issues with a hard idle... But one thing added on, my idles jumps from 1000-2000 rpm's at least every two seconds... The surge is killing me... And it backfire and runs harsh as well.. I am currently working on it. I just had to rebuild the DME and I also own a Oscope... I do believe its something to do with throttle sensors. I had another car that did the same thing, and one TPS took care of it... I will let you know my findings..

Also check your grounds. That can be a symptom of erratic gauge behavior.

I also second checking the speed/reference sensors. Checking those without an oscilloscope is hit or miss. Without seeing a graphical analysis of the voltage output you can't be 100% sure

[QUOTE=Porsche-O-Phile;4025801]WIN FREE PARTS FOR CORRECT DIAGNOSIS!!!



-Idle is a little bit rough (kind of like maybe one cylinder is intermittent?)
- Fuel efficiency/MPG gauge (which is normally all the way to the right at idleoccasionally "twitches" to the left in conjunction with the rough idle/misfires.
- When you put your foot on the accelerator all the way down (WOT/floor it), the engine will bog down and "pop" loudly once or twice or possibly stall out completely. In any case, it feels like it isn't good and probably not a great idea to continue doing.
- If you put your foot about 1/3-1/2 way down, the RPMs will come up, but very slowly/gradually. The engine doesn't quite sound right - more like a Honda lawnmower than a Porsche.
- Extremely difficult to drive due to lack of power/engine not wanting to "spool up" & roughness/misfiring occasionally
Here's what I've checked so far:

- Confirmed fuel pressure at the front of the rail (flows when key is "on")

- Put some Seafoam in the fuel to loosen up any crud


All these things to me point to fuel pressure. You said that you had fuel at the rail, but if the regulator isnt maintaining proper pressure during engine operation, then you will feel all these symptoms of poor idle, slow to accelerate, lack of power ect. Also my other thought is the timing is a bit off either due to a timing belt that jumped tooth or too, or maybe the rotor is not tightened down or rotor is somehow not positioned properly. I would look at fuel or timing. Most people have way more 944 experience than me here, so I am sure they have good suggestions, but as a tech on all different kinds of cars I would check timing and fuel pressure

you have one or more bad injectors, or possibly something to do with the sparkplugs firing (cap/rotor/plugs/wires). you can confirm by pulling and replacing one ignition wire at a time while the engine is running (if you are brave), or by pulling one and starting the engine. when you pull a wire and there is no difference, you have found the culprit, although there may be more than one. my friend's 944 would actually run on two cylinders and sounded/behaved exactly as you described. two of the four injectors were not firing at all and were found to be bad.

I think its the DME.

I say it is catalytic converter. Just disconnect the header and test it.

Jeff, it is real easy just to pull the distributor cap and check and make sure that everything is tight and right underneath. I'd start there and be certain that the rotor is secure, the cam bolt tight, and the contacts all clean before getting into the more involved procedures.

LOL! I forgot about this thread - I have a similar problem with my Benz (exact same symptoms) and I couldn't remember offhand what the problem was with the 944, so I came on here to check. It was the speed/reference sensors. Car runs great now. I owe someone a free something-or-other whoever said it first...

Tps and or vac leak.can you hear a click when you open the tb by hand.

Nah it's long fixed. It was a bad speed or reference sensor (I changed 'em both). The repair was extremely simple actually. I'm in a similar situation with my Mercedes right now so I've got to go and get a code reader to figure out which sensor has gone bad. Too many silly-azz sensors in cars these days. I wouldn't be the least bit surprised if it ends up being whatever the corresponding sensor(s) are in a Mercedes SLK to the Porsche 944's speed/reference sensors. Virtually the same exact symptoms, which is what led me back to this thread... Anyhoo - gotta' go get me an OBD2 reader.

Moral of the story - if your 944 engine is running like poo, check your vacuum lines, your distributor, your speed/ref sensors. :)

so what do i win !!!!!

Holy thread resurection..............

Other than the 'pop' these are my symptoms. Have tried an exhaustive list of fault finding but no joy yet.

I thought that with a faulty speed/reference sensor the car wouldn't start. I have tested as per Clarks but no Oscilloscope. Just trying to sum up the 'collective will' before playing parts lotto.

Thanks,

M

What did you conclude from this thread, maybe you should start a new thread on your specific issues...

I was just hoping somebody out there had similar issues and was spurred on by this thread to fix it. I have tested a fair number of parts on my car recently and whilst the sensors appear OK as per Clarks bar the oscilloscope, having read this old thread this is what I am experiencing.

Just hoping someone can confirm this is the likley culprit before I spend more money on parts!

Thanks,

Michael

OK...so..managed to get sensors out...not too bad.....the one nearest the bulk head in hole 'D' was badly eroded on the top as in it was rubbing against metal. I gaped new sensor as per Clarks. I stuck an 8mm thick cut up credit card to gap them. I cranked the engine. Straight away the tacho jumped up to around 4500 on start with no foot on accelaration...first time I have ever seen that...exciting, engine felt much more alive.....however there was this high pitched metal grinding whine. Sure enough I checked new sensor and the new one was eroded on the top as well. It turns out the aftermarket one is slightly...maybe...1mm longer than the OEM.

There does not seem to be the amount of play on the up down motion of the bracket on mine against this instruction vid https://www.youtube.com/watch?v=-MbghejYiqk

I have now put the old one back on as I have no more!

Convinced it is the speed sensor that is the problem. So question is, how hard is it to gap and if the sensor was scratched eroded would this make the difference? Was a bit adverse paying top whack for OEM Bosch sensor so bought these instead https://www.ebay.co.uk/itm/CRANKSHAFT-S ... 2749.l2649

Thoughts please.

Thanks,

M

I can't see what you bought sensor wise but people were running bmw ones at half the price of a porsche ones. The only difference are the plug in lines are longer.