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Calling 16v 944S no start troubleshooting geniuses
OK, to start, I’ve read about every “no start” thread on the internet and followed all the steps in the Porsche “944S Test Plan DME” manual I have the means to and still, no start. So here I am looking for advice on how to proceed as it seems I’ve overlooked something.
Background: I bought a non-running ’87 944S 16 valve 2.5l (poor choice on buying a "previously ran" car, not the issue right now). The PO stated he had previously rebuilt the engine (he bought it with a spun rod bearing), put about 25k miles on it then it would idle but not rev (would sputter and die with throttle). It then sat for upwards of five years best I can tell. It wouldn’t start when I looked at it.
I brought it home and:
-drained the fuel
-replaced fuel pump (Bosch)
-replaced fuel filter (Meyhle)
-replaced DME relay (Pelican)
-put in fresh fuel (premium)
-new battery (Napa)
-verified fuel pressure, 36psi with pump jumpered, holds pressure when turned off
Still no start, not even with starting fluid. My inductive timing light fires.
-replaced cap and rotor (Bosch)
-replaced plug wires (Clewett)
-replaced plugs (NGK Iridium)
-replaced and gapped speed and reference sensor – only one on an S (Bosch)
-swapped DME with known good
Still no start.
-checked injectors, two appeared to be frozen.
-Sent injectors off for cleaning and flow test, all in spec now
-checked compression, very low in Cylinder 1
-removed head, all 8 exhaust valves bent (best guess, guides were replaced without cutting seats to match)
-rebuilt head, new exhaust valves, valve job, surfaced
-noticed some wear and light scoring on bores, not ideal, but not a show stopper
-reassembled; new tensioner pads, belts and bearings (Lindsey kit), water pump (Geba) (evidence suggests these parts were not replaced during “rebuild”), cams set up with dial indicators per service manual, compression now 185-200 in all cylinders
-verified engine harness; replaced several sections of wire, replaced connectors, checked against wiring diagram
-replaced ignition module (Bosch)
-checked timing through window in cam housing, fires around 0 degrees (I used a dab of my daughter's silver glitter paint so there may be a margin of error)
Still no closer to starting than the day I brought it home.
It seems to have fuel and spark. As of late the plugs have been wet when I pull them. I blow them off and blow out the cylinders before trying again. My starter was weak and eventually died, I’ve since replaced it and am using a big Optima from my other car to turn it over.
To address some of the common issues:
-I've cleaned and checked all the grounds
-I have about 1-2 mm of tach bounce and my oscilloscope shows 3.46VDC at the DME connector, I don’t see a decisive reference mark as in the manual but I can see the -2 waveform
-A smoke test did not reveal any vacuum leaks
-I’ve tried two DME relays and a jumper lead (made per Clarks Garage)
-The alarm module is removed and bypassed
-Air Flow Meter shows 0.25VDC at rest, increasing in a smooth and linear manner to a max of 4.25VDC, it does take about 1cm of movement to leave the 0.25VDC mark
Aside from basic tools, I have at my disposal: the full set of factory service manuals; DME troubleshooting guide; digital oscilloscope; multimeter; timing light; fuel pressure test set; air compressor
Great first post huh?
I look forward to your insight.
I did recheck compression and now have 185-200psi in each cylinder. I do not have a leakdown gauge setup, so no data, but I surmise the cylinder wear will leave it below normal. Do you think that would cause a no start condition? Thanks for taking a crack at it.
I did recheck compression and now have 185-200psi in each cylinder. I do not have a leakdown gauge setup, so no data, but I surmise the cylinder wear will leave it below normal. Do you think that would cause a no start condition? Thanks for taking a crack at it.
Good, I was just checking as you didn’t say and no, excessive leakdown shouldn’t cause a no start.
So are you getting anything or is it just spinning on the starter? With fuel and spark I’d expect some sputtering maybe? You haven’t by chance swapped the speed and reference signals? It certainly seems like you’ve covered a bunch of stuff and it may be something easy you’ve overlooked.
Occasionally, on my first try of the day I get a little sputter. I got close to an actual start once but a little feathering of the throttle killed it. It's an "S" so there is only one sensor for speed and reference. Here's a picture of what I get on my scope. The waveform does not quite match what the manual says; this looks like it captures the "-2" signal, but not the "reference" mark which the manual illustrates as a spike above the regular voltage. I've even tried turning the key with my fingers crossed while saying "klaatu verata nictu". I'm kind of at a loss.
I've read your entire list and what sticks in my mind is the AFM or more specifically its connector and/or wiring. My only clue is what was happening as told by the PO: "...it would idle then not rev". This is what happens when an AFM is disconnected.
Other than that, I'd say you have a very thorough and impressive list there! Double check you've put the wires in the proper order
Occasionally, on my first try of the day I get a little sputter. I got close to an actual start once but a little feathering of the throttle killed it. It's an "S" so there is only one sensor for speed and reference. Here's a picture of what I get on my scope. The waveform does not quite match what the manual says; this looks like it captures the "-2" signal, but not the "reference" mark which the manual illustrates as a spike above the regular voltage. I've even tried turning the key with my fingers crossed while saying "klaatu verata nictu". I'm kind of at a loss.
The AFM passes checks, but is borderline witchcraft even new, I ditched it in favor of a MAF setup on my old turbo Ford. The no voltage increase until a mm or so of movement is mildly concerning, I'll need to consult my Bosch book on that. I verified the wiring is good. New speed sensor is Bosch from AutohausAZ (not genuine Bosch.cn or some such sketch).
For reference, here is what the manual says the signal should look like. There is no scale here (just "higher amplitude") and it is as measured with "special tool", not a lab instrument.
And wires on the "crab cap" are ordered 3-1-2-4 from top to bottom. That is, from everything the internet tells me, correct.
Also, three screw rotor, not the slip prone single set screw type.
You tested the AFM on its own or plugged into the car? That is, did you test the connection and/or wiring? Perhaps test it at the DME end. The rest is far beyond me, that's for sure. Hopefully, some 16V experts will offer help. Very interesting issue though. I agree that the "Ref. mark signal" signal you measure does not seem to be as dramatic as the manual suggests. It also appears to be the opposite phase.
And wires on the "crab cap" are ordered 3-1-2-4 from top to bottom. That is, from everything the internet tells me, correct.
Also, three screw rotor, not the slip prone single set screw type.
...3-1-2-4 (top to bottom) is incorrect see pic below and have you also confirmed they go to the correct plugs, 1-2-3-4 with 1 at the front?
Edit: to be clear, looking at the front of your distributor and reading clockwise the leads should go to 1-3-4-2 in that order.
That is the picture I used for reference. I interpreted it as the numbers on the cap illustration representing which cylinder the wire on that terminal goes to. It is entirely possible I misread it and that would certainly lead to this kind of problem. Here is what I have, yellow traces for clarity.
03-22-2019, 04:38 PM
Calling 16v 944S no start troubleshooting geniuses
