Background: 1.7 liter which has been upgraded with new pistons and cylinders, original fuel injection. Car started and ran, then stalled and would not restart. Covid hit so it sat for a while. Now at mechanics shop where they have replaced the fuel pump (rebuilt original) and the manifold pressure sensor (rebuilt).

Engine has low manifold vacuum, 10-12" and will not start. If they apply 15-18" of vacuum with a vacuum pump it starts right up and will run fine for 20-30 minutes (on pump), then stalls. If they attempt to restart, it backfires and will not restart until cold.

Any suggestions on what to check or what next step should be?

1) All High Pressure hoses (the ones on the plenum) - replace, also check that hose connector (see attached 5&13)
2) if that does not work, remove plenum and check all of the welds, they rust and then leak. I used 2 stage epoxy as a fix for my plenum - not optimum but works!

See half way down in this post for schematics by JEFF BOWLSBY, item # 5&13

http://www.914world.com/bbs2/index.php?sho...57345&st=20


This is for a 2.0, though it is a DJet, should be a good place to start.

Do as previous post suggested and have mechanic do a smoke test on the vac system. Who rebuilt the MPS? Was it calibrated for your engine? Did they do a vacuum test on the MPS?

Start with a compression test.
If you pass that test, it’s just a matter of replacing hoses.

Stop throwing parts at it.

I’d fire a mechanic just throwing parts at a problem. Seems that he’s guessing and that you’re paying to educate him.

Check cylinder head temp sensor resistance vs ambient temperature per Anders page referenced below:

https://members.rennlist.com/pbanders/

Parts table:

https://members.rennlist.com/pbanders/DJetParts.htm

Almost all Djet components can be bench tested. Throwing parts at it and guessing is unacceptable IMHO.

Compression first...

What year? Which ECU?

To respond and answer some questions. We are not throwing parts at it. Both the fuel pump and the sensor were checked and found to be inoperable before they were replaced.

The car is a '72. When the engine upgrade mentioned was done a used wiring harness with ECU was installed. I assume it was a used "original" part but cannot confirm.

Compression test was done and good on all cylinders.


While not a Porsche specialist mechanic does have experience with air cooled engines and works frequently with older vehicles.

I agree with Superhawk above
Send this link to the mechanic

https://members.rennlist.com/pbanders/

AND
Remove and tape over all other vacuum hose connections to the intake plenum.
If it runs ok with JUST the MPs hooked up for vacuum, start replacing one system at a time.
I would suspect the AAR because they corrode open.


Ensure the fuel pressure is good on both sides of the regulator

Here’s the resistance vs ambient curve for the various Cylinder head temp sensors.

A 1.7L would should be using the 012 sensor
Click to view attachment

I agree with Superhawk996. Vacuum is higher at idle, so pulling extra vacuum is pushing the ECU more towards ‘idle conditions’, which means less fuel. So it won’t start normally because it’s too rich and the extra vacuum brings it lean enough to start, but it’s still too rich and eventually bogs down. This is also why you get a backfire when you try to restart. And why it won’t restart till it’s ‘cold’ (really you’re waiting for the extra fuel to dissipate). And that also means you’re washing down the cylinder walls of any lubrication.

The first culprit is likely the CHT as mentioned. Or the CHT wiring. I would check the resistance <i>ideally at the ECU connector</i> — or disconnect the CHT and wire in an appropriate resistor (I think it might be around 10k ohm) and see if that does anything.

Additionally the rebuilt MPS might not be appropriate for your setup and may be curved wrong, but I doubt it’s wrong enough to cause this much issue.

Good luck.

Welcome back Mr Kotter.

nope

Thats pretty low manifold pressure but intensely retarded timing could result in very low vacuum, that mixed with some vacuum leaks could result in your issues. May want to verify timing visually (quick and easy) while you are digging through the hoses. You don't even need to have the MPS vacuum connected for the engine to run. I have accidentally started mine many a time with the MPS vacuum disconnected. In fact, you can pull the TPS, the MPS, the plenum temp sensor, etc and still get the car to start and idle. The CHT and MPS need to be wired to the ECU for it to run though.

need more info, if your mechanic cannot figure it out, have him call me, they all do

Check the ignition system first, then make sure the valves are in adjustment and you have even compression.

Check for unplugged wires and hoses. Or stuff that's not plugged into the right places.

Then move on to your next troubleshooting steps.

--DD