Carpokes Porsche Forum

Hi Folks - just when I thought I had my 87 S all buttoned up, I'm getting a massive hesitation between 3,700 and 4,000 RPMs. It's a major hesitation, where it feels as though the DME is cutting ignition. The car runs fine at lower RPM's and there are no issues with starting.

My DME temp sensor is about a year old and I replaced the connector, so I don't think that's the issue. My TPS clicks as it should and ohm readings check out at idle and 2/3 throttle. I did discover that when I replaced the FPR and Damper last year, I ordered and installed 3.0 bar FPR instead of the proper 3.5 (or is it 3.8?) bar FPR that the 16v's use. Of course, the 87 and 88 have their OWN FPR's (barb fitting instead of threaded), so it makes it even more confusing.

I ordered the proper FPR and it should be here the end of the week. My question is, if the car has lower fuel pressure than the DME is expecting, could this cause the DME to pull spark at higher RPM's?

Regarding your specific question, I'd say the lower max fuel pressure would be the more likely issue at higher rpm. The DME is unaware of fuel pressure, it just assumes the correct pressure is there. The only thing that would cause the dme to pull out spark lead in this case is if there's engine knock (the sensors can pick up knock we can't hear). A compromised spray pattern from the injectors would make the issue worse, as well.
This kind of performance thing can also be caused by bad plug wires, carbon tracking in the boots, etc, especially if this is something that just started happening out of nowhere. Or maybe you have a fuel filter starting to plug up...

icb wrote: Tue Feb 06, 2024 8:51 am Regarding your specific question, I'd say the lower max fuel pressure would be the more likely issue at higher rpm. The DME is unaware of fuel pressure, it just assumes the correct pressure is there. The only thing that would cause the dme to pull out spark lead in this case is if there's engine knock (the sensors can pick up knock we can't hear). A compromised spray pattern from the injectors would make the issue worse, as well.
This kind of performance thing can also be caused by bad plug wires, carbon tracking in the boots, etc, especially if this is something that just started happening out of nowhere. Or maybe you have a fuel filter starting to plug up...

Thanks Ian - I appreciate the help with this. The plug wires are pretty new - about 2 years old but less than 1K miles on them. Plugs are about a month old and I verified the gap prior to installing. Fuel pump and filter both have less than 1K miles on them. I'm praying the proper FPR fixes the issue. If not, then I'll be back to square 1 with testing.

Just an update (I hate open ended threads) - the issue still persists despite installing the correct FPR. Because the issue mainly happens when the car is warm, I'm looking at the following:

DME temp sensor and harness (both were replaced but will check and verify)
TPS - Facet unit. Clicks when it should, reads as it should but I haven't tested readings when warm - only cold. Replacing with Bosch unit.
Flywheel sensor - just because I still randomly get the "starting quicker than it should" issue. Also Facet and replacing with a Bosch unit.

I hope to have time this weekend after my turbo gets picked up for paint. I'll report back with my findings.

Just a thought, Dennis. It could be many things, but ultimately it should reduce to either an ignition or fueling issue, as you know. Maybe it'll shed some light to warm it up, find a deserted straight stretch of road, then take a hard long acceleration pass and shut it off immediately when the issue occurs. Unlock the steering, pull over and pull the plugs to see if you can read anything there, ie wet plugs = ignition cutting out, dry lean plugs = fueling issue, that kind of thing. Or the plugs may show nothing at all, but I think it would be worth while to take a look, especially if you're getting a really big hesitation.
My gut feeling is that somehow, it's something simple that's getting overlooked, since it started all of a sudden.

icb wrote: Tue Feb 20, 2024 1:53 pm Just a thought, Dennis. It could be many things, but ultimately it should reduce to either an ignition or fueling issue, as you know. Maybe it'll shed some light to warm it up, find a deserted straight stretch of road, then take a hard long acceleration pass and shut it off immediately when the issue occurs. Unlock the steering, pull over and pull the plugs to see if you can read anything there, ie wet plugs = ignition cutting out, dry lean plugs = fueling issue, that kind of thing. Or the plugs may show nothing at all, but I think it would be worth while to take a look, especially if you're getting a really big hesitation.
My gut feeling is that somehow, it's something simple that's getting overlooked, since it started all of a sudden.

Thanks Ian! Unfortunately, finding a long stretch of deserted road here in Raleigh is getting hard as more and more people move here. My gut is telling me it has something to do with the TPS. The problem doesn't seem to happen if I slowly bring up the RPM's to 4K - it's when I give it higher throttle and reach those RPMs.

I have read a few posts online where people are getting the "click" at 1% throttle and proper resistance at 2/3 throttle, but when replacing it the issue went away. I've been burned by aftermarket parts before (Chinese ICV, URO items, etc) so I'm hoping a genuine Bosch unit will do the trick. I'm going to replace the reference sensor with a genuine Bosch since the intake will be out AGAIN. I'm thinking of doing a timelapse video to see how quickly I can remove the intake assembly. I think I can do it in less than 20 minutes at this point!

Do you get a loud backfire after it cuts out? That would suggest ignition is cutting out and fuel is still pumping. No backfire suggests fuel (and possibly spark) is cutting out. Does you tach drop to zero when it cuts out? That would point to ignition and maybe dme or speed/ref issue. Just clues, nothing conclusive. How are the cap and rotor, coil, plugs, grounds, and wires? Cuts outs under load are often caused by weak ignition. Have you tried watching a spark plug arc to see if it 'looks' ok? Could be a million things of course, including bad aftermarket parts, so just food for thought. Adding some logging tools scan go a long way to help diagnose these issues too...

Tom wrote: Wed Feb 21, 2024 7:52 am Do you get a loud backfire after it cuts out? That would suggest ignition is cutting out and fuel is still pumping. No backfire suggests fuel (and possibly spark) is cutting out. Does you tach drop to zero when it cuts out? That would point to ignition and maybe dme or speed/ref issue. Just clues, nothing conclusive. How are the cap and rotor, coil, plugs, grounds, and wires? Cuts outs under load are often caused by weak ignition. Have you tried watching a spark plug arc to see if it 'looks' ok? Could be a million things of course, including bad aftermarket parts, so just food for thought. Adding some logging tools scan go a long way to help diagnose these issues too...

No loud backfires and I didn't notice any drop in RPM's. When it happens, I typically go into "oh please don't leave me on the side of the road" mode and baby it back to my house, park it, crack a beer and swear a lot. With that being said, it idles and drives great at lower RPM's.

Wires/cap/rotor all less than 2 years old and maybe 500 miles on them.

Literally the only things that I have not replaced in that engine bay is the ignition ballast, rod bearings and piston rings. I'm also going to check for any vacuum CLOGS. I already found that one of the URO "Y" vacuum fittings was clogged right out of the package. That was for the HVAC system though.

I really can't wait for Joe to develop an F9 DME for the 16V cars. I have one on my 951 and it makes diagnosis MUCH easier with the ability to log while idling or driving.

You probably wouldn't be wasting your time taking a closer look at the TPS. I've opened/cleaned/serviced mine twice. They are difficult to open without ruining, but possible of you go slowly and carefully. It helps lower the anxiety if you have a good spare. The first time was during a throttle body rebuild, and it was full of oil from the leaking lower throttle shaft seal. The WOT switch is an open switch, literally, unlike the enclosed micro-switch of the idle contact. Any oil in there has direct access to the WOT contact. It's hard to see in this photo, but the WOT contact is right at the top and slightly left. As a side note, carefully check the solder joints inside; an invisibly cracked joint at the idle switch (the left one in this shot) was the cause of a very hard to track down high-idle-when-warm condition that haunted me for almost two years.

I had an S22 that did that. On mine it was the flywheel sensor.