914/6 w/a stumbling issue ideas req's'd

[9146INSC]

I know that this is the 911 forum, but I have a 914/6 w/a 95' 993 engine and 915 combo that has a (RECENTLY) only hiccup/Stumbling problem that is cropping up during a drive. It's been doing it for a few months now. First it was just a blip or hesitation, but now each drive is getting more pronounced and noticeable.

A little background to help. Engine had a full rebuild on 2010. I have put about 3000 miles on it since with no real issues. Starting, Driving, or putting the hammer down. The car starts and drives smooth and steady every time. No issues what so ever with the starting and driving.

I drove it the other day, in this miserable heat to go to he beach, and about 30 miles into the drive, the car starts hiccupping. An intermittent misfire.. I found I could control it somewhat by modulating the throttle. It only does it under load. I turned the car around, and headed back home hoping We wouldn't get stranded. We didn't.
An Ease up on the throttle diminished the misfire. There was no loss of fuel pressure. I have a red dash light to indicate power loss to my fuel pump. Light never comes on.

My first thought was the excessive heat working against the electronics, which at the time were mounted in the engine compartment. Infared heat gun showed about 150 on the ECU & Ignitor. I've since moved the electronics to behind the seat, and today the heat gun say's about 95-100 in their current location. Much better but the car still hiccupped again today. I probably drove about 20 miles before it did it again. Oil temp was about 200 degrees. Not to bad, that is were it usually is. Outside air temp is around 95-99.
I hooked up my Duratech when I got back home, while idling, and no fault codes logged.
I also ruled out wiring, as everything was a "plug and play"install. 993 Engine wiring loom was changed a year ago.
I'm wondering if I'm having a component failure Due to the excessive heat we've all been experiencing lately.

All systems are working normally, when the vehicle is cold, and still working smoothly up operating temperature. The stumbling has been screwing with me for a couple of months now. Steady drive down the interstate, smooth as glass, then a period of misfire for a while and then it goes away, only to return when it decides to.
Any ideas??? maybe vapor locking?, coil going bad ?Ignitor failing? Any one BTDT, and what was the culprit. Any advise or ideas would be apprecited.

Ron

[Joe Bob]

Have you thought about fuel delivery? Fuel filter or the strainer in the tank?

Also, do you have a volt meter that you can see if there are dips in voltage during said hiccups?

[9146INSC]

Originally Posted by Joe Bob

Have you thought about fuel delivery? Fuel filter or the strainer in the tank?

Yes, Fuel delivery was attended to last year. New tank, Aeromotive pump w/a 10 micron inlet filter, all stainless fuel hard line from front to back. Lastly a 1 micron Canton 6'' filter mounted on the firewall just before the engine inlet.

Also, do you have a volt meter that you can see if there are dips in voltage during said hiccups?

Good idea there, I've got one of those volt meters that plugs into the cigar lighter. I'll definitely run that to see what happens.

[Joe Bob]

Those plug in thingies are nowhere near accurate, but you are just looking for spikes....

Does the 993 have a DME relay and use Motronic like the 964s?

If so, a hiccup could be an intermittent disruption in the 12v power supply. Check the fuse in that circuit, a bad solder in the DME, a loose DME relay.....or any number of things.

Can you get the hiccup when sitting or just under load?

[9146INSC]

Originally Posted by Joe Bob

Those plug in thingies are nowhere near accurate, but you are just looking for spikes....

Does the 993 have a DME relay and use Motronic like the 964s?

Yes, Motronic, and the DME relay. The engine/chassis Wiring interface was made by Patrick several years ago. It has worked well. I never had to cut any wires to install. Just 4 tap ins.

If so, a hiccup could be an intermittent disruption in the 12v power supply. Check the fuse in that circuit, a bad solder in the DME, a loose DME relay.....or any number of things.

Can you get the hiccup when sitting or just under load?

Hiccup only occurs when the car is under load I just took it out for another 15 or so miles with the volt meter in the cigar lighter, and no hiccup, or spike/drop in the meter the entire time.

[Joe Bob]

Well....wait until it happens again. Try swapping out the DME relay....if you don't have a spare, you should.

They are notorious for sudden fails. Intermittent glitches are usually in the DME themselves.