adammillsap

I've continued to work through issues with Linda (1987 944s), I've replaced all ignition components from the coil to the cylinders and confirmed good spark on all 4 with a spark tester. I've followed the timing procedure for the cams to the cam gear, and found that the timing was off by 10 degrees or so. I now have the TDC mark on the flywheel aligned with the mark on the cam gear, and when those are at TDC , the cylinder 4 tappet is depressed 1.4mm. The car now idles. Still a bit rough, but I can turn the key, and expect it to idle. It also chokes when I try to rev from idle. If I go easy, it revs smoothly once I get above 1500 or so. When I shine a timing light on the cam gear at idle, spark occurs about 3 degrees before TDC. As I rev the motor, this moves earlier.

The rough idle led me to investigate further, and I discovered that cylinders 2 and 3 were not firing. Checking exhaust header temps, I see 1 and 4 at about 400-600 degrees, and 2 and 3 o only at 120 degrees. I confirmed spark with a spark tester, and appear to have strong spark on all 4 cylinders. The injectors were cleaned and flow tested by an injector refurbisher last week. I did a compression test, and found all 4 cylinders with compression right at 135psi, and they all hold steady for 15 minutes or more. This leads me to think I may not be getting fuel to 2 and 3.

I don't have a great understanding of the relationship between the DME, the ignition module, the spark plugs and the injectors, and haven't been able to find an explainer on how those systems work together. Can anyone give me an explanation of what mechanisms are involved in timing injection and spark? How do the various sensors play into this? Any common failures that would lead to this condition?

Thanks a bunch!

walfreyydo

The injectors on these car operate in "batch fire" (meaning they fire in pairs for both the exhaust and compression stroke for the given cylinder pairs). Given that two cylinders are suspected of not getting fuel, I would assume they are part of a pair. The first thing I would do is measure voltage at those injectors and see if they are getting a signal. If no signal I would suspect issues with the injector wiring harness and do some further inspection of the wiring.

See the section on Clarks Garage for "Other Fuel Related Issues" on how to test the injector signal (of course I am sure you have this site bookmarked and have thoroughly read through the shop manual section... right?)
https://clarks-garage.com/shop-manual/fuel-16.htm

And this page for engine troubleshooting guide
https://www.clarks-garage.com/shop-manual/ts-01.htm

Zirconocene

I'm sure that your harness is pretty crispy at this point. If you do any investigation, be as gentle as possible. On my '91 there were breaks in the wire sheathing right where they bend from the harness to the injector. I wasn't having issues but I don't think that I was far away from issues, either.

Here's my understanding of how spark and ignition work: The DME uses the CPS to determine where the pistons are. Based on that information, it decides when to fire the injectors. The DME fires the injectors by grounding the circuit, which is why it's really important that your grounds are in good shape. On my '91 there are 2 grounds that on the DME harness in the engine bay, one on the block and one on the bellhousing. If you haven't cleaned them up yet, you should do that and see if it makes a difference.

The other thing you can try is to pull the injectors with the fuel rail and try to start the car. Put the injectors into some plastic cups and see what happens. This can be done after the much less invasive testing from above.

You can buy replacement harnesses from various places but it's also not terribly difficult to replace this particular run on the harness, if you're so inclined.

Good luck