924Board.org

tomcooper · Joined: 17 Feb 2004 Posts: 301 Location: Waterloo, Nebraska

Ladies and Gentlemen,

A little assistance solving a mysterious problem with my NA 924 if you please. It’s a 1980 and started acting up on the way to Effingham for the Porsche Funfest in June. The problem is a miss at speed after the car reaches operating temperature. When cold or cool the car runs fine, but after about fifteen to twenty minutes a miss starts and continues to get worse as I drive further.

When in Effingham, I purchased a new set of injectors and installed them. On the way home from Effingham the fuel pump relay burned out and was replace. Thinking that the main fuel pump was causing the problem that was replaced. After reading several threads about the in tank fuel pump and the problems it can cause, I drained and flush the tank, (as described by Vince Ponz) replaced the in tank fuel pump and installed a new fuel filter Still no solution.

This evening Jazzerpet and I took compression readings and they ranged from 195 down to 175 psig. That should be well within limits for the valve train to be working properly.

Has anyone else run into a similar problem where the car runs fine when cold, but starts to miss after running for about fifteen to twenty minutes. I would appreciate any help in solving this problem.

Thanks,

Tom Cooper, tcooper40@cox.net

Paul · Posted: Wed Aug 10, 2005 1:40 pm Post subject:

Let's try to figure out if it is an ignition/fuel problem or a mechanical problem by hooking up a vacuum gauge and taking it for a ride until it starts to miss.

Tell us what the vacuum gauge does each time the motor misses. If it ticks with each miss, it's probably a valve problem, if it doesn't, it's time to look at the ignition and fuel systems.

Khal · Posted: Wed Aug 10, 2005 1:46 pm Post subject:

All my genius mates, who fancy themselves as Gordon Murray-esque in their mechanical engineering brilliance (can you tell I'm being sarcastic), always offer the same sage advice... "It's either a fuel problem or a spark problem."

Never mind that there's a gazillion different causes for the fuel/spark problem...

Anyways, seems you've thoroughly eliminated the fuel system from the list of contributing factors. That leaves the spark...

Nothing comes to mind that would obviously cause the symptoms you're describing. But the majority of the "consumable" ignition parts i.e. dizzy cap, rotor, points -does the '80 NA have points?, plug leads, plugs, etc (save perhaps the ignition coil) are pretty cheap. If those parts haven't been replaced for a while it might be worth replacing them just to eliminate the possibility..?

Sure does sound heat related to me, though. Y'know, something heats up, expands, breaks contact, causes intermittent miss...

Although I've learned that the ol' toofahs have their fair share of quirks, so I await the advice of the real experts who may seen this sort of thing before

EDIT: I wasn't havin' a go at you Paul

I'd actually submitted my reply before I saw yours! And I never even thought of hooking up a vacuum gauge... can you tell who's the mechanic?!

_________________
'80 924 Turbo

leadfoot · Posted: Wed Aug 10, 2005 6:23 pm Post subject:

Yeah normally it's the spark betwwen the two ears that's the problem....

I'm going to go with a problem in the electrical system though.... and here's why....
A fuel problem would cause an issue at warm/cold temps but moreso at cold as combustion becomes easier at hotter engine temps, the flame front ignites easier that is and causesa preignition if the ignition is to advanced.

Therefore I would suggest checking the basics such as the resistance form ignition cables i.e to the coil and transitor unit, check the contacts on the dissy rotor cap as these wear easily and build up copper oxide on them(these are also really cheap to replace), and if you haven't replaced your leads recently consider doing this too, as I have had rough idle before and the resistance checked fine but when I replaced the leads it cured the problem.
Also I say to most people check your spark plugs as they are the best indication of how your cylinder are operating (besides a vaccuum gauge which is also pretty handy). Look for oil deposits, lean mixture, rich mixture and check that the heat range is right for your operating range.
Leadfoot
_________________
1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

Paul: Where's the best place you've ever found to hook up the vaccum gauge on the 924?
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'80 924 n/a SOLD

Vince Ponz · Joined: 02 Nov 2002 Posts: 3581 Location: Florida

I can't stand this constant bickering.

If you don't know the joke ask me.
_________________
"Never let them see you sweat"
77.5 924 modified track car
79 931 Euro stock
88 924S SE
87 911 Targa stock

tomcooper · Joined: 17 Feb 2004 Posts: 301 Location: Waterloo, Nebraska

Okay, to answer a few questions that I did not cover in the first post. The cap and rotor are new about six months old. The wiring is new about the same vintage. All the smaller vacuum lines have been replace leaving only the large braided lines untouched. The ignition system is breaker less.

Paul, your suggestion of using a vacuum gauge will be my next step. Somehow I think that this is a small problem that can be easily fixed, the hard part is to isolate the exact component that is causing the problem.

Thanks for all the ideas.

Tom Cooper, tcooper40@cox.net

Chrenan · Joined: 15 Jan 2003 Posts: 3903 Location: Canada

Tom, I've never had the problem you have had, but I have replaced all the items you list. Since you've gone this far, you might as well go the whole distance and replace the coil and large braided vacuum lines, they are both cheap compared to what you've replaced. Could it be anything simple like a plug wire too close to the exhaust manifold and it only being effected as the egine warms?
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1987 951 - M193 Version for Japan

Jazzerpet · Joined: 07 Apr 2003 Posts: 186 Location: Omaha, NE.

It could be just that simple and that is something we have not thought of. The crew has been working on this problem off and on since we went to Effingham. We have a vacuum tester that could be rigged and read inside the cabin fairly easy. I should have a spare coil laying around the shop to switch out for a test. These are all good ideas guys keep them coming.

While we are at it there is a second problem this car has and it seems to be contagious. Sometimes when the key is turned the car will only click, sometimes it does nothing at all unless the key is held in the start position for a few seconds. One of the cars had a starter swap, no change. Tomorrow evening we are going to put one of them on the lift and rebuild the harness to the starter. The large positive cable has been replaced already. The car tends to turn over a bit slower then it should as well.

Anything we should look at wile we are at it. TIA
_________________
1980 931
1985 ½ 944
Member of the “Omaha Crew”

Jazzerpet · Joined: 07 Apr 2003 Posts: 186 Location: Omaha, NE.

Double post.
_________________
1980 931
1985 ½ 944
Member of the “Omaha Crew”

Benino · Posted: Thu Aug 11, 2005 1:39 am Post subject:

This is a part in the distributor that you may want to replace.
http://www.autohausaz.com/catalog/lookup_parts.cfm?SubcategoryID=7185&VehicleID=1263259&CFID=521200&CFToken=35864680

on this site, it's called an Impulse/Pickup Coil. I have heard it referred to by another name, but I can't remember. It's a little bit of work to get the old one out of the distributor. You could just install a distributor from another car to see if maybe your Impulse/Pickup coil is going bad.
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1980 Porsche 924 N/A USA
1980 Porsche 924 turbo USA
1987 Porsche 944S USA

Paul · Posted: Thu Aug 11, 2005 8:38 am Post subject:

Tim Cuddihy · Posted: Thu Aug 11, 2005 10:01 am Post subject:

I've got a mate that had that problem with a 911. The plugs were the wrong type I think they were too hot so it was all good until it reached operating temperature then pre-ignition took over.

crazyinkc · Joined: 10 Nov 2002 Posts: 226 Location: Olathe KS

My 77 ran good cold but would mis under load. Not a consistant mis, just here and there. I found the insulator on the #3 plug was cracked. Four new plugs and everything was fine. Just a thought.

tomcooper · Joined: 17 Feb 2004 Posts: 301 Location: Waterloo, Nebraska

The "Omaha Crew" did a vacuum test on my 924 this evening, and it would appear from that we have a fuel or ignition problem. The vacuum holds steady when the engine misses. Since most of the fuel system is new, everything but the fuel dizzy, the next place I plan to look is the distributor.

Jazzerpet loaned me a spare distributor, and I plan to take it to one of the local Porsche Specialist to see if it is working properly. Then I'll swap it out with the current disribtor and see the result.

Incidentally when we did the compression test the plugs looked good.

Thanks for all the ideas.

Tom Cooper, tcooper40@cox.net