PerryB

Several years ago, I replaced the speed and reference sensors as part of a clutch job. After bolting everything back up the car refused to start. What happened next was a 14 month long journey into L-jetronic/Motronic diagnosis that led me to replace/rebuild a lot of parts unneccessarily trying to fix the car. It turned out that one of the replacement sensors was defective, despite passing all test. This sensor was manufactured by a company called FAE. The outer shield on the sensor end that was grounded against the bell housing also grounded one of the pins on the connector end. The computer didn't like this and refused to fire either the ignition or the injection. Again, the sensor passed all the diagnostic test called for in the factory WSM and in several other troubleshooting guides.

If either sensor is FAE brand, bin it and get OEM Bosch.

M4A3E2

Hi! Sorry for the lack of updates, life got in the way.
ran a compression test using an autozone compression gauge and my results are not pleasing me. Low compression in cylinders 1 and 3, both at 65 PSI.

Chalt

What were the compression #s on all four cylinders?
#1 65 psi
#2
#3 65 psi
​​​​​​​#4

peanut

You've twice stated in previous posts that the engine compression is good yet it is now clear that you never actually checked this !...

You ask for help but then don't respond to suggestions from experienced fellow owners and then disappear for weeks ......?

You've randomly replaced 12x components without testing them or following a sensible troubleshoot sequence and we are expected to diagnose why the engine won't run.

your low compression could easily be wrong ignition timing, incorrectly set followers, incorrectly cut valve seats or valves not seating properly lots of things but probably you've screwed up somewhere
.Such a low compression is at least unlikely to be pistons rings .
Please tell us what all the readings are and PRECISELY how you made the tests. I'd also appreciate a reply to my suggestion to check the ignition components post #30
Always best not to assume anything but double check to be certain

M4A3E2

1: 65 PSI
2: 120psi
3: 65psi
4: 125psi

I understand. Before my compression *seemed* good and I’ve been in a way lately, regardless that doesn’t excuse behavior.

peanut

thats ok ....it can get a bit frustrating at times but bottom line .....everyone on here wants to help you.

How many revolutions did you crank the engine for each compression test ? was it an equal number of times ?

Try this simple test.....pour a tablespoon of fine bicycle oil into one of the low cylinders and spin the engine over a few times to allow the oil to coat the cylinder walls and rings.
Then do another test on that cylinder and see if there is any change in compression.
Sometimes too much fuel into a non starting engine can lead to temporary bore wash

M4A3E2

I turned the engine over for about 15-20 sec each time.
I’ll do that when I get home! Also bringing home new speed/reference sensors today.

walfreyydo

If doing a compression test, crank the engine until the needle on the gauge stops increasing. There is no specific number of revolutions or anything, just crank until you get the maximum reading.

Also, compression test results have nothing to do with ignition timing. Its 100% to do with the sealing of the cylinder, which is influenced by valves (ie: bent), valve seats, piston rings, cylinder bore health

peanut

its not about the revolutions but many owners have no idea about how to carry out a compression test.
Too fewer number of cranks will not produce the maximum compression
A varying amount of revolutions could also produce varying results .Too many unnecessary revolutions for 4x consecutive cylinder tests like the 20+ of the OP is way too many and puts an unnecessary strain strain on the battery and associated cables .

For your information compression has everything to do with correct ignition and camshaft timing .
If the camshaft was not correctly aligned with the crankshaft when the OP rebuilt the cylinder head then of course it could effect the running of the engine as valve timing will be out and valves possibly not fully seating!.... Not likely its true but still a possibility to eliminate

M4A3E2

Slight delay on things, waiting on the sensors to arrive. I will do compression tests again when I have the new sensors.

peanut

.
just a note of caution...before changing them are you certain that both the sensors are faulty ? I wouldn't have thought that you needed to touch them or most of the other stuff that you have changed when doing a head gasket job .Here is an excellent tutorial of their function and testing procedure
If they aren't faulty , changing them is risking introducing yet another possible fault into the equation.
i don't know how much all the new parts have cost to date but it must be considerable.
Its much better to test and check things logically first so that the number of components and settings that you effect are reduced to a minimum.

Could we perhaps hear the engine starting and running again if it will that is.

walfreyydo

Cam timing yes, ignition timing no

M4A3E2

I would love to send a video of it running again but it won’t start. That’s kind of my main issue haha
I ordered new sensors as no matter how I have them plugged in I have tach bounce… which seems odd to me

peanut

I think you are misunderstanding my text. I'll try to make it clearer
Cam and valve timing can effect compression . If the cam timing is off that can also effect correct ignition .
Hopefully that makes things clearer

Chalt

I would like to add that you can rent a leak-down tester from an auto parts store. Since you have 65 psi compression on #1 and #3, you need to find out why the readings are low. Replacing the speed and reference sensors is a separate issue from low compression readings. A leak-down test on each cylinder will help you identify what exactly is leaking. The test will pinpoint if you have:

1. Leaking intake valve
2. Leaking exhaust valve
3. Worn rings (leaking past rings)
4. Leaking head gasket (blown)

Sure, you could Not do a leak-down test and pull everything apart and just start replacing everything. But knowing precisely where failure has occurred prior to disassembly is valuable information.
If you choose not to systematically and methodically test things, you will always be in guessing mode and that's expensive and unnecessary because you are spending money on parts you didn't need and your time on repairing and/or replacing parts that were good.
When you get lost, slow down and test, test test. Also, document, document document your tests. Write down dates, times, what test readings were. Make notes about what you worked on and tested. You can also use your phone camera or video to help you document test readings and such. That way you will know for sure what you have done or not done. Troubleshooting without documenting is a scattered/ shotgun approach. When you test methodically and record results, you will be able to communicate to yourself and others what is going on.

Here is a pretty short generic leak-down test video (with annoying music). It explains the how and why for a leak-down:


Here is a bloke doing a leak down test on a 944:


If you want to learn more about leak down testing, search it on youtube and/or a search engine.