Ran great for the 1st DE of the season but on the way home...

[DaddyGlenn]

Last weekend I did my first DE of the season. I had some great instruction and am already miles ahead of where I was last season. This was at a new track for me. And a new sticker for the car.



The car ran great for the 3 1/2 hours to the track, back and forth to the hotel and through all 8 sessions. However, about halfway home, cruising down the highway, she bucked once. Like like a misfire but no back fire or other noise, just a split second loss of power. Then a few minutes later, she did it again. It got more and more frequent. I kept going.

There were no other indicators of anything wrong. Coolant and oil temps were good and oil pressure, too. Nothing in my voltmeter, either. It seemed to worsen with more throttle so I kept a light touch, loosing speed on the uphills and letting gravity help me get my speed back going down.

Through the next hour or so it got pretty bad, bucking every 1-30 seconds. It didn't matter if I was in 4th or 5th gear, the more I asked of the engine, the more it complained.

Here's the odd thing, it was at its worst just before I pulled off the highway but for the 5-10 minutes from the highway to home, not a singe issue. I even tried to make it happen. She took full throttle up hills without complaint.

More, that may or may not be related ... a few times when I started the engine before a DE session, I could hear the fuel pump whining loudly. It would eventually stop and didn't happen every time. Also the bucking started about 5 minutes after I had stopped to fill the tank. So my first thought was the issue on the way home was related to fuel.

I haven't checked fuel pressure and delivery yet, but it is first on my list.

Looking for others that may have had a similar experience and resolved it or recommendations of ways to identify the cause.

Next day on track is scheduled for April 23rd so I have a few days to figure it out and fix it.

Thanks

[mytrplseven]

Fuel supply hose to fuel pump kinked or collapsed, fuel pump, electric connector to the fuel pump, fuel filter, water in the gas, tank filter plugged, etc., etc.

[onZedge]

I recently did some preventative maintenance by replacing all the ceramic fuses and cleaning the fuse holders with Deoxit. Even though I never had any fuel pump related issues, the fuel pump fuse "element" crumbled when I pulled it out. It may have been a problem just waiting to happen. This is perhaps the easiest thing to check first.

[DaddyGlenn]

Fuel pressure is 32psi at idle. I've got to make a jumper to measure it with out the engine running. And I'll have to get the gauge connector to stop dripping to get a good leak down.

But I'm already thinking it is not a fuel flow issue. From my research, for it to be a fuel flow issue, I believe there would have to be an over-pressure caused by a faulty pressure regulator. 32psi is within spec.

But I'll keep checking.

[grendiers]

Check the in-tank fuel filter. Also, the secondary fuel filter, or just replace it. The in-tank filter used to be pretty cheap, now they're up to 70 bucks I think. If gunk is passing through the filter, it will take out your fuel pump.

[Jfrahm]

Could be water in the gas. If it clears up for a while when you add some Heet or 91% isopropyl alcohol that's a good clue. It might keep coming back (less and less) for a while though. It's troubled me in the past.

There is a good Project Farm video on youtube about water in gas.

[mch3jsb]

Could very well be your afm. I had one that would act up after an hour of highway driving, swapped it out with one of my spares (suggested as a must have by a couple 944 spec guys). Search here and also some good info on Clark’s about checking these.

[DaddyGlenn]

Update: Fuel pressures are good and the AFM checks out just fine, too. I am assuming it was water in the gas. I've drained the tank and will add fresh gas and hope for the best.

Lesson learned: Don't fill you tank with premium fuel from a station in the middle of frickin' nowhere. They probably don't sell much premium so the gas has probably been sitting in the underground tank for ages.

[ewalt98]

My first thought was bad gas. I had nearly identical symptoms on a 500 mile drive in my F150. Had to feather the gas to keep from bucking/hesitating for most of the drive. Thought for sure it was a problem with the coils, which can be difficult to diagnose if intermittent. Had a dealer look at over a few days in Oregon and they could not identify any hardware issues. After a second tank of gas, the issue was completely cleared.
I later talked to 2(!) friends who had the same issue with gas from the same place, which is a high volume station just outside of Missoula. Needless to say, I have never gone back to that station.

[DaddyGlenn]

After the tank was drained, I added 5 gallons of fresh gas and it started right up. I ran it for a few minutes and shut it down. The next morning, Wednesday, it wouldn't start. I started basic troubleshoot and found good fuel pressure (still) but no spark. I then ran the "no spark checklist" and found a bad crank speed reference sensor. It must have been going bad and intermittently not signaling the DME causing the DME to cut off power to the ignition

I had a SCCA Track Night in America scheduled for that evening and, of course, nobody stocks those sensors locally. So I took the 911, on well worn street tires and crappy brake pads, for an evening of easy track driving and socializing. It was still much better than not going at all.

Since I have Watkins Glen next weekend and wanted to be sure I had time to sort out any other issues, I ordered 2 new sensors from our host, paying extra for delivery today.

New sensors are now in and she fired right up. When it stops raining (tomorrow), I'll take her for a good test drive.

So put that one in your notebooks, a failing speed sensor can cause intermittent engine bucking.

[mazdaverx7]

That's good to keep in mind should it ever happen. I always try to stock a set of speed/reference sensors. I appreciate you following up on the cause of the issue!

[Peter Graham]

I had a similar issue with our 912 coupe driving to/from Texas Hill Country Rallye. It turned out to ignition related: in this instance it was a Pertronix ignition module. I cannot say what it could be with a 944 but checking the ignition would be next after ruling out fuel system...

+1 on checking crank sensors/connections: our 951 has an occasional no-start condition due the sensor connector. Wiggle the connection everything works for a couple months...

Peter