85 911 Starting issues

[Afrusciante]

Need your help Guys.

My 85’ 911 has starting issues. Go down to pull her out of the garage for a ride and it starts right up. Stop at a store, run in and out. Jump back in the car, put the key in the ignition and all the car does is turn and not fire up. You wait 5 minutes, sometimes more and the car fires up. Nerve racking to say the least. You never know what’s going to happen when you turn the key.

The car has 112,000 miles, has been completely tuned up and new DME. Can this be a crankshaft sensor issue?
Thanks

[boyt911sc]

Check your fuel pressures (with & w/o vac.) including residual pressure. Your FPR (fuel pressure regulator) or the FP check valve could be the culprit.

Tony

[gomezoneill]

I think he has a point here. I would suspect the sensors if they haven't been replaced with that mileage.

Get a flashlight and look under the left rear fender on the bell housing there. Visibly check the insulation and condition of the two sensors. Not a fun job to replace them if they're corroded even on a lift. I just replaced mine and it was a bear.

[GH85Carrera]

If the crank sensors are old or if you don't know when or if they were replaced, just do them and the O2 sensor "while you are in there" because they don't last forever.

Can you hear the fuel pump run for a second after you turn the key? If not that can be your issue. The DME relay usually fails with some warning and while running will start to run bad.

The crank sensors are something to replace even if they test out OK. My last set of sensors would OHM out just fine, but evidently failed when they were hot. Replacing them cured my 85 running issues.

Good luck with the hunt for a fix. I love my 85 911, and I drive it a lot. After 174,000 miles it runs like a Porsche is supposed to. I will keep driving it to other states until I hit all of the lower 48 in the continent. Just 8 states to go for my 911.

[darrin]

Afrusciante - this sounds like the same situation I ran into when my DME relay was failing (it had developed cold solider joints that caused the self-resolving no-start issue) -- know you indicate above that your car has a new DME (the computer), but do you know whether the relay behind it was also replaced? If not, I'd start there

[Stutzdriver]

Swap out the DME relay under the front seat, oh, you don't have one? Big mistake as you should ALWAYS have one or convert to the solid state one. The original type are notorious for being an issue, ask me how I know. The solid state replacement can be had with the fuel pump "prime" where the fuel pump runs a couple of seconds before the car cranks unlike the original that cranks AFTER it gets the "rotation " signal form the flywheel. Starts quicker every time!

I replaced the sensors on the flywheel when one went bad. I used the BMW part number for less than 1/2 the cost of the Porsche #, I just had to deal with a longer wire which was no problem.

[Afrusciante]

Thanks guys for the suggestions. I’ll go down the line and look at each.

[Nick Triesch]

Take off the plate that covers up the fuel pump up front. Take a rawhide mallet and tap the electric pump. Then start it for several times. If it is much better, replace the fuel pump for $200!

[Nick Triesch]

Or an aftermarket for much cheaper. I bought Bosch

[Afrusciante]

Thanks Nick. Ordering the Bosch FP. Could be the best $200 I spend.

[RedCoupe]

Don't ignore Tony's suggestion about fuel pressure. I wouldn't go changing a fuel pump without testing fuel pressure first and verifying there is a problem. Also, a faulty check valve could bleed off pressure and make starting difficult.