Carpokes Porsche Forum

I recently had my head rebuilt and did all of the work associated with replacing the head gasket. The car has been sitting for about six months. Everything is back together, and now the car won't start. It will crank, and kinda sounds like it wants to start here and there, but won't actually run. It ran before I did all the work, so I'm trying to figure out where I screwed up.

Timing is confirmed good: the flywheel mark and cam/rear cam housing mark line up perfectly at TDC, and the distributor rotor is pointing to the cylinder 1 point. The cam mark is about 1 tooth off relative to the front cover mark, but I assume the rear cover is more reliable?

I have spark, confirmed on all four cylinders. I have checked and rechecked that the plug wires are in order on the distributor.

Compression is 170-175psi for all four cylinders.

I haven't checked fuel pressure at the rail yet, but have in the past and it was good. All four injectors do put out fuel. I haven't checked to see if they're all flowing the same yet, but will the next time I work on it. The gas in the tank is probably 2-3 years old, but I did add stabilizer to it, so hopefully that helped a bit. Trying to run the car with one injector disconnected didn't make a difference. I tried disconnecting each one individually. There is no gas in the vacuum lines of either the FPR or dampener. I also got under the car while it was cranking, and the fuel pump seems to be working.

While the car was sitting, I also adjusted the AFM wiper arm based on the FR Wilk instructions, and confirmed that it was putting out a good signal after I did that. With a 9V battery, 0.163V when closed, 8.0V when all the way open. Smooth increase in voltage throughout the range. Air temp sensor resistance was 2.658kohm at 66F.

I don't remember touching the speed/reference sensor wiring when I did everything, so their connectors shouldn't have gotten swapped around by accident. They both have good resistance based on the Clark's Garage specs; I can check them with an oscilloscope if need be.

I checked all of the vacuum lines when I had the intake off, and everything is in good shape. From what I can see currently, they are all hooked up.

The starter solenoid signal on DME pin four is 12+ volts. I also saw a tiny movement of the tach at times when cranking it, but the tach used to stick anyway, so I can't say if a lack of movement there would be conclusive.

I tried to start it with starting fluid, which resulted in a massive backfire out the intake. From what I've read, this would indicate an ignition timing issue? I just don't know where that would be coming from. I also had the exhaust off the car while it was sitting, and a new cat welded in, so maybe the exhaust is blocked somehow? I guess a rodent could have moved in there, but I didn't see any obvious evidence of that.

Ultimately, I'm stumped. My next steps are to check fuel rail pressure and injector flow, and I guess to pull the exhaust and see if that makes any difference. Any help or advice would be greatly appreciated!

Backfiring through the intake might suggest the an intake valve is open when the plug is firing - either due to cam timing or ignition timing. Did you check the distributor rotor to make sure its not spinning free on the cam -- that's relatively common and could explain why it seems like your timing is off despite everything else you checked.

How do you know all 4 injectors are putting out fuel? Did you see them squirting out gas?

If you know there is fuel, and you know there is spark, and you know there is compression, it almost has to be cam or ignition timing -- unless one of those things isn't right despite your checking. You might try unplugging the AFM in case its sending such a bad signal the car can't run. Checking fuel pressure is a good idea, and getting new gasoline is a must, but none of that would really explain backfiring through the intake.

How confident are you in the head work? If an intake valve isn't closing right, I suppose spark could ignite the starter fluid in the intake that way even if the timing it ok. Though, you'd expect that to show up in the compression numbers.

Injector connectors can and do get frayed under the connectors, and if one shorts out, none of them will fire. One the other hand, if you pulled plugs and can see/smell fuel squirting in, that's probably not it.

If you have a spark pulse, it's not likely related to speed and ref sensors. Maybe post some picture of the timing marks and plug wires and such in case we can spot something you're overlooking?

If you are concerned about excessive belt tension, there are sprays you can apply to the belt to increase their friction with the pulleys, I have no experience with them, but they are available.

There are two TDC's: one on the compression stroke and one on the exhaust stroke. Are you sure it's timed to the compression stroke?

The rotor is fixed to the cam, the screw that holds it is installed and properly threaded into its hole.

I put some ziplocs over the injectors with the fuel rail removed and cranked it, and got gas in all four. I plan to do the same with cups next time to be able to see if they're all spraying relatively evenly. If I see any issues I'll peel the boots back and see how the wiring looks.

I'll try to run it with the AFM unplugged and see what happens.

I don't think there's any reason to doubt the head work, but I guess you never know. The head shop did find an issue with the cylinder 3 exhaust valve guide that was making the valve stick open, but they fixed that. And like you said, I would think that any issues would show in the compression test, like they did before the work was done.

Regarding compression vs exhaust TDC, if the cam and flywheel marks line up when cylinder 1 is at TDC, it must be on the compression stroke, right? Since the crank goes around twice for every rotation of the cam, wouldn't the cam be off by 180 degrees at compression TDC? I'll post some pictures if I don't get it running next time I work on it.

Thank you for the help.

boing wrote: Tue Jun 09, 2026 8:20 pm...
Since the crank goes around twice for every rotation of the cam, wouldn't the cam be off by 180 degrees at compression TDC? ...

Yes, that sounds correct. I only had this issue on a Ford Kent, which has no exposed cam gear.