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

[Porsche-O-Phile]

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.

[looneybin]

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

[Rasta Monsta]

. . .and their plugs.

[John_AZ]

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.

John_AZ
1988 924S + 1987 924S

[many944s]

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

[Slam]

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.

[Slam]

Again with the double post! Sorry.

[nynor]

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.

[AKCJ]

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.

[Porsche-O-Phile]

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?

[nynor]

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.

[Porsche-O-Phile]

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.

[many944s]

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

[nynor]

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

[Porsche-O-Phile]

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

[Rasta Monsta]

Originally Posted by many944s

you will drop what ever tool you use to remove those d@mn allen screws

And blood. . .lots of blood.

[951Boost]

Originally Posted by nynor

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

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.

[zedsn]

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

[legion]

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.

[bholmes]

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.