Carrera 3.2L Rev Limiter Problem - Speed Sensor Solution

[Bristol]

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:

3.2 carrera rev limiter

3.2 l stumble at 3K plus

1988 911 Carrera @+-4000RPMs under load, power loss/break-up

Carrera won't rev over 5800 RPM

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.

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

[Tippy]

One question, how did the gap get off in the first place?

BTW, good to hear you found the issue!

I can tell you that the gap can be pretty big without effect - at least on mine.

[djcdmv]

I hope this solves my problem...

For those who find this thread years later, here are some other answers from a thread I posted a few months ago.

Help! 3.2 looses power at 5500 rpm while accelerating,

Tough my issue is STILL unresolved, I'm hoping the speed sensor is my answer.

[Bristol]

Originally Posted by Tippy

One question, how did the gap get off in the first place?

Good question. I meant to address that. I just got the car. The previous owner changed the clutch and could not get it to run right afterwards. They got frustrated and sold it, cheap... They must have moved the bracket during the clutch job and did not adjust it right. They also destroyed the shifter linkage during the same clutch job.

Kirk

[DRACO A5OG]

wow , bad for PO good for you

[Joe6pack]

I want to say a big thanks for this thread. I just finished my clutch job and though I replaced both sensors "while I was at it", I was hitting the 4,000 rpm wall. After another hour under the car adjusting the speed sensor, she runs like a scalded cat.

Thanks again.

[Chuck.H]

Thanks for the post, sounds like good investigative work there!

One thing that may help this diagnosis is to look what the tach is doing during the studder; I know when my speed sensor went bad, I got a bad miss and the tach dropped down several thousand RPM's in milliseconds, which of course is impossible when it's in gear at oh lets say the speed limit, with the transmission turning the engine whether its firing cylinders or not.

Chuck.H
'89 TurboLookTarga, 338k miles