Pelican Parts Forums - Carrera 3.2L Rev Limiter Problem

The purpose of this e-mail is to document a significant run issue that I had with my car (’86 Carrera 3.2L) and how I resolved it so that others, in the future, may not have to search as hard as I did to find a solution to the same problem.

THE PROBLEM/SYMPTOMS
The car ran normally at lower RPMs, except for some slight hesitation when lugging it. As the revs and load build though, with acceleration, the car will simply hit a wall. It feels like the car is hitting the rev limiter, but at a much lower RPM than where it should hit the limiter. In my case this varied from 3,500 to 4,000 RPMs, but other Pelican posters report it happening anywhere between 4,500 to 5,800 RPMs. When the car hits this “wall”, it just suddenly starts missing hard (it is not progressive, but rather abrupt) like the engine is being fuel starved and every cylinder is missing (not just one). Another Pelican post described this problem by the “bah, bah, bah, bah” sound the car makes as it’s missing. Applying more throttle will not allow you to power through the bad spot and I hit the “wall” whether accelerating gently or WOT. Other than this issue, my car cruised and accelerated fine.

THE SOLUTION
I did a bunch of searches on Pelican over several weeks and replaced the DME relay, fuel filter, plugs, wires, cap, rotor, coil, and battery along the way. All were in need of replacement anyway, so no big deal. When this still didn’t resolve the issue I did some more checking but not replacing – fuel pump, fuel tank ventilation, sensor measurements, mass air flow sensor, fuel pressure regulator, vacuum lines, etc. I didn’t want to just keep throwing parts at this problem and did some more searching on this board. Here are some of the key posts from five to seven years ago that I finally found by hitting on the right search terms:

http://forums.pelicanparts.com/porsche-911-technical-forum/172349-3-2-carrera-rev-limiter.html

http://forums.pelicanparts.com/porsche-911-technical-forum/186685-3-2-l-stumble-3k-plus.html

http://forums.pelicanparts.com/porsche-911-technical-forum/180145-1988-911-carrera-4000rpms-under-load-power-loss-break-up.html

http://forums.pelicanparts.com/porsche-911-technical-forum/70785-carrera-wont-rev-over-5800-rpm.html

These posts suggest that the speed sensor is the key problem. In my case I found that the speed sensor was fine, but the gap between the sensor and the flywheel was too great. When I reset it to a gap of exactly 0.8 mm my problem was completely resolved. The car will now rev to your heart’s content and as a bonus it runs smoother at lower revs when you are lugging it. It’s like a completely different car, and a ton more fun. In my opinion, I believe that with the wrong gap the speed sensor was “lying” to the DME about the speed of the flywheel. The speed sensor signal must have been such that the DME calculated that it was approaching rev limiter speeds, but at a much lower actual speed. So I believe what felt like a rev limiter that I was hitting at 4,000 RPM was really, truly the rev limiter from the DME kicking in at what it thought was an appropriate engine speed.

I really want to thank Pelican for running this board and thank the great people on here. If in doubt… search, search and then search some more. :D

Kirk

'03 Carrera 4S - Techart
'00 Boxster S - Gemballa
'86 Carrera Targa - slant nose wide body
'75 911S Targa - project, beater car
'72 914 - Chalon slant nose wide body