midnite993

Hi All,

I've been driving my newly acquired '95 993 sold to me by a fellow Rennlister. After losing my original '96 993 to a red light runner, this car has been a blast to drive until this past weekend.

The car now takes about five turnovers to start in the morning, whereas before it only took one at the most. After the first start of the day, the car will restart normally (one turnover).

If you drive it like a grandma and only use part throttle, the car behaves normally. If at any RPM you push the gas more than half way, the car will stumble and buck like it is being flooded, and if you let off the throttle it will backfire. Idle is smooth as glass, and you can rev the engine to redline in neutral at any time without stumbling, so the problem is only under load. This occurs whether the engine is cold or hot.

I read up on diagnostics here, replaced my DME relay, and I think I've eliminated ignition issues by:

1. Disconnecting the low voltage leads to coil for top distributor to check for broken dist. belt. The car runs the same way (stumbling under load).

2. Running the top distributor from the coil pack of the bottom distributor to check for a bad coil. Car runs the same way.

Cap and rotor can't be the problem because the car acts the same way no matter which distributor the engine is using. Check engine light does not come on at all. Can the Bosch hammer scantool read anything useful from the '95 OBD1 ECU?

The fuel pump and alternator were replaced by Desert European Porsche (dealer) in Palm Springs just before I bought the car. The car ran awesome for the first week I had it...so it would be strange if something they did caused this. My next step is to replace the fuel filter which should arrive tomorrow.

I'd appreciate any help...I was looking forward to driving it to Fresno on some sweet back roads this weekend.

vjd3

Bad throttle position switch? (Assuming there is one on the 993.)

I had a similiar problem on my old 3.2 Carrera.

ppashley

Sorry, you say replaced the ECU relay. Are you referring to the DME relay?

1pcarnut

Beautiful car, sorry to hear this. I bet your thinking "just my luck!" Anyway, DME relay maybe although this is more go/no go. Fuel starvation or air leak would be next approach.

Derrick B.

I don't have my manuals handy, but there in Chapter 8 there is a table that relates sensor failures to symptoms. The Check Engine light is primarily an emissions checker, so there are probably several failures that would show a fault when read, but not cause a CEL. Two of the culprits I can think of are the RPM sensor, Hall Effect sensor. I think that fuel pressure is a good starting place. Any smart people out there?

Hopefully someone can break out the manual for you...

Great looking car by the way.

midnite993

Yes, the DME relay was what I was referring to. I replaced it with a new one that I had not yet installed on my '96. Thanks for the manual reference...I have access to these.

One other thing I forgot to mention is that the exhaust smells strongly of fuel when I first start it up in the morning.

Thanks for the compliments on the car.

NP993

Well, but what about the distributor drive belt? The twin distributors aren't like the twin fuel pumps in an old RSR, where one serves as a backup. They both have to be functional.

BobbyT

Actually, you don't need an OBD reader to read the codes on a 95. Here is how:

1. Turn the ignition on, but don't start the engine.
2. Depress the accelerator pedal for approximately 3 seconds. The CEL will illuminate steady, then begin to flash.
3. When the CEL begins to flash, ease off the accelerator.

The flash fault code will consists of four digits such as 1121. Each digit will be identified by a number of flashes followed by a pause. For example, flash pause flash pause flash flash pause flash stop means 1121. Flash pause flash flash flash flash flash stop means 1500, no fault codes stored.

After you read the codes get back to me and I'll look them up for you.

+1 ppashley--the DME relay, located in socket #53 in the front boot fuse-relay panel, controls fuel pump power, and is a known trouble source. Some of the factory-installed relays were a 944 part, which should be replaced with the improved relay, part #993 615 227 00. My guess is that this is the source of your problem.

Nice car--mine is identical, even down to the wheels, but in Guards Red.

ppashley

Suggest you read the full thread entitled "My 95 wont rev over about 5400....Help!! "

It might be useful.

midnite993

Noah, I figured the distributor drive belt was OK since the car will run from the secondary distributor (lower belt driven one) alone. Driving it set up like this exhibits the same symptoms.

Bobby, Hey...I live very close to you in San Bernardino! I've also owned similar older Porsches in the past. We should meet up sometime. I will try the CEL trick you describe. I already replaced the DME relay as I mentioned earlier.

BobbyT

It still might be worth trying another DME relay. They deteriorate over time.

You have a PM.

95FL993CAB

how about bad gas?
did you change the fuel filter?
Ron

allill

My 96 did the same thing as far as backfires and stumbles when its flywheel was going out and sending error codes. Not sure if it's the same for obd1 though.

midnite993

Some new developments. I went out to rev it up again in neutral, this time really flooring it. If I floor it at idle the car will rev to 4200 and just stick there running rough! If I let up on the gas a bit it will then jump up to redline. So it's doing it all the time, not just under load like I thought. Does this narrow the problem down a bit or eliminate any possibilites?

I read through the CarreraX thread (car won't rev above 5400) and will try removing that bolt behind the O2 sensor and measuring the pressure as in this post:

https://rennlist.com/forums/showpost...&postcount=164

This will test the cat flow, which ended up being CarreraX's problem. Would be strange if the cat went bad, it passed a tough CA dyno smog check with flying colors just last week!

After that, I plan to measure the fuel flow aft of the fuel fiter as per Steve Weiner's recommendations:

https://rennlist.com/forums/showpost...&postcount=101

Will then wait for my new filter to arrive tomorrow and will install it as its due for one anyway.

VNTGSPD

Just a WAG, but have you checked the throttle position sensor (TPS) or the mass air flow sensor (MAF). Have you tried accelerating at different throttle positions while under load or have you always been giving it full throttle?