1986 911 won't rev

[Tim_in_MD]

Hello all,

My '86 911 (all stock) is giving me issues. It didn't get used much last year and towards the end of the summer, I took it out and it wasn't revving very enthusiastically and sounded like it was only hitting on four cylinders. It had occasionally exhibited this behavior before, generally if it had been driven very short distances without warming up and/or with lots of idling, and would sort itself out after a very short distance of at-speed driving. However, this time it did not.

It got stored all winter without me digging into it (I know, I know). About a month ago I got into it and it was hard to start, idled low and unevenly and would not rev, even with half-throttle. It was as if the gas pedal had no effect. I drained the tank as that gas had been from November 2017 (though it was stored with Stabil at that time). I changed the oil and replaced the plugs with new Bosch; the old ones were a little dark/wet but not terrible. With 5 gal of fresh gas, new oil and new plugs it behaved the same -- catches weakly on a long crank to start, idles lumpy and low and wants to stall, won't rev at all on the throttle. At most it'll do 2500 rpm with a big throttle opening and that's barely enough to roll it back into the garage.

I tested the speed and reference sensors and found the speed sensor to have resistance above the acceptable range. I replaced both with Bosch part 0 261 210 002. The car still exhibits all the same symptoms.

Where do I go next? Coil was tested as good last summer (in the course of chasing a different problem on an '85 Targa). Cap and rotor aren't new but don't have more than 15k miles on them. Same with plug wires. Battery was new last summer and is always on a tender.

What about fuel pressure? The pump is original on a car with 159k miles and has occasionally exhibited symptoms of a bad check valve. Now, it'll pop a little bit with the big throttle openings...not a loud backfire but a softer pop noise...as if the mixture is lean. Still, the exhaust smells like normal start-up rich.

I've owned this car for almost 12 years and know it pretty well. This is the first head-scratcher it's thrown at me in 20k miles.

Thanks in advance for any help.

[carreradpt]

Tim, my first thought with a 3.2 is to replace the dme relay. Then go from there.

[cabmandone]

Could be fuel related. There is a test procedure somewhere on here for the fuel pressure regulator. I think the regulator is located on the front side of the drivers side intake manifold. Could also be a failed cylinder head temp sensor.

[Pedro '84 Coupe]

Check the CHT sensor. Also, when was the last time you changed the fuel filter?

[john walker's workshop]

See that black spot on the center coil terminal? That's a burn-through that arcs across the black outer covering that 3.2 and later cars use. Cross-fire-o-rama. No evidence of tracking inside the cap. Had to cut the outer shell off to see.

[proporsche]

i know you probably did you reference sensors replacement correctly -but the q is have you measured the distance from flywheel?
0,8mm

[Tim_in_MD]

Originally Posted by carreradpt

Tim, my first thought with a 3.2 is to replace the dme relay. Then go from there.

Good idea -- will do that. I think I have a spare around to swap in.

[Tim_in_MD]

Originally Posted by cabmando

Could be fuel related. There is a test procedure somewhere on here for the fuel pressure regulator. I think the regulator is located on the front side of the drivers side intake manifold. Could also be a failed cylinder head temp sensor.

I'm wondering about fuel pressure too. Will find that procedure and see what's happening there.

[Tim_in_MD]

Originally Posted by Pedro '84 Coupe

Check the CHT sensor. Also, when was the last time you changed the fuel filter?

Two votes for CHT sensor. Will test that. Would that affect cold start?

Fuel filter was done a few years ago -- at same time as all ignition stuff. No more than 15k miles on that too.

[Tim_in_MD]

Originally Posted by john walker's workshop

See that black spot on the center coil terminal? That's a burn-through that arcs across the black outer covering that 3.2 and later cars use. Cross-fire-o-rama. No evidence of tracking inside the cap. Had to cut the outer shell off to see.

Have had a heck of a time with coils on other cars. This one tested OK last year but I will pull it and check for arcing. Thanks!

[Tim_in_MD]

Originally Posted by proporsche

i know you probably did you reference sensors replacement correctly -but the q is have you measured the distance from flywheel?
0,8mm

Yes--great question. I found a .8mm-thick ring connector in my pile of stuff and glued it to the end of the old sensor. Used that to reposition bracket, installed the new sensor. When the car still didn't run right, I pulled the new sensor, loosened the bracket and used the old sensor with spacer to set it again. Still didn't work. I'm pretty comfortable I have that where it should be...

[john walker's workshop]

The coil wasn't causing the arc, the high tension burned through the cap.You can't check that area without cutting the black shield off the cap.

[cabmandone]

Originally Posted by Tim_in_MD

Two votes for CHT sensor. Will test that. Would that affect cold start?

Fuel filter was done a few years ago -- at same time as all ignition stuff. No more than 15k miles on that too.

Yes it would. The CHT plugs in where the speed and reference sensor plug in. Make sure the plug is fully seated. I accidentally bumped mine and the car ran terribly until I noticed the plug was slightly askew. I plugged it back in and the car ran fine again.

[Tim_in_MD]

Hello all,

This is a long-overdue follow up to my original post from August. I've been picking away at this when I've had time. I think I'm down to the CHT sensor but it's an odd failure.

To recap, here's the situation on the '86 911, stock everything, 160k miles. Will crank and fire but immediately drop to a very low idle and stall. Second hit of starter does same, on third hit it will stay running but at a very low idle. Revs do not increase with opening of throttle and it will start to pop back through intake if throttle is held open. This got progressively worse during the fall of 2018 until getting to this point in the spring of 2019.

Here's what's been done.

Replaced plugs with new, no change.
Inspected cap and rotor, no obvious issues.
Drained tank of old gas and put in five gal of fresh, no change.
Replaced speed and reference sensors (insulation was cracked), no change.
Checked fuel pressure, 34psi at rail and held 22psi after 20 minutes.
Swapped coil with known good, no change.
Swapped ECU with known good, no change.
Replaced DME relay, no change.
Pulled the ICV, cleaned it and checked for hum, no change.
Pulled AFM to bench-test and check for grooves in carbon sweep, moved arm and got perfect results with 9V battery test, no change upon reinstall.
Tested intake air temp sensor and got 1.9 ohms at 80 degrees ambient.
Tested CHT sensor and got 1.92 ohms at 80 degrees ambient.
Tested the idle switch and it ohms out properly and gets 5V from ECU.
Checked vacuum hoses in engine bay, all fastened and no visible cracks.

Here's where it gets weird: I disconnected the brake booster/cruise vacuum line to isolate that as a source of vacuum leak and in the course of doing so inadvertently knocked the CHT plug loose. I started the car and it fired right up and held a high idle, gradually working up to about 2k rpm.

I noticed the CHT plug was disconnected, reconnected it, and the car went back to the original failure mode of stalling, super-low idle, and no throttle response.

I reconnected the vacuum line and unplugged the CHT sensor, and it went back to a high idle, etc. I drove the car around the neighborhood and as it warmed up, the idle started to hunt a bit, but it stayed running and firing on all cylinders.

So here's the question: it seems like the CHT is the culprit, but it also seems to ohm out OK. Over the summer, I got an appropriate 1.92 ohms at 80 degrees ambient, and today with a garage at around 55-60 degrees I got 3.169 ohms. That looks like it's in range.

Does this sound like a reasonable result from a bad sensor, or is it masking something else? I will throw a new sensor at it, but with the Bosch version NLA at all the vendors I'll be spending $280 on a Porsche version and would like to hope I'm not barking up the wrong tree.

Thanks for any insights. If the answer is to follow the obvious path, duh, that's great.

[darrin]

is your CHT a single wire unit (original to my '86) or the newer 2 wire unit? When my single wire CHT failed, my car wouldn't start and disconnecting the CHT didn't rectify or change that situation. If you still have the single wire unit, it might be a good idea to proactively replace it, regardless of whether it's the root cause of your current problem.

Also, your troubleshooting writeup above doesn't seem to address/rule out John Walker's distributor cap suggestion -- as he indicated one wouldn't see that the cap was shot (and allowing the high voltage current it supposed to direct to internally arc around) unless one cut the cap apart. Since you were able to swap both a coil and an ECU with known good ones, why not do the same with your distributor cap as your next step?

[Tim_in_MD]

I will do that -- should be able to borrow a cap from the same source as the ECU and coil. Helps to have a friend with a disassembled 911.

[Tim_in_MD]

Today's update: got my hands on another distributor cap and swapped it on, did not solve the problem. I did cut the cover off my old cap and indeed found two small burned spots on the tower, much as John Walker suggested, so a replacement has been ordered.

I have ordered a new CHT sensor and will report back once that is installed.

[stlrj]

Originally Posted by Tim_in_MD

I have ordered a new CHT sensor and will report back once that is installed.

Meanwhile, I would go ahead and replace the ignition cables since ignition problems are still not out of the question.

[mysocal911]

Originally Posted by Tim_in_MD

I have ordered a new CHT sensor and will report back once that is installed.

Once the engine is warm and the temp sensor is below 500 ohms, it has little to no effect on the running of the engine.
You can just jumper the two pins of the sensor with a paperclip when the engine is warm to eliminate it.
It only provides for fuel enrichment when the engine is cold.