85 94 running on 2 cylinders.Solved!

Hi All, I have another mystery. I have been cleaning up the dodgy factory wiring  on my 85/1 and while I was at it decided to change the ref sensors and injectors which I got from Ian at 944 online. Today I went to restart it and it is only running on 3 and 4. There is spark at 1 and 2. The injectors for 1 and 2 are clicking. Fuel pump is fine. Fuel at the fuel rail. Total mystery. The car ran fine before I brought her in for the electrical upgrade. 12v at each injector. Spark at the spark plugs. Ref and Speed sensors the right way around. I did not have to gap them as I did not touch the bracket and the old ones looked fine. I am stumped... I have used a fibreoptic camera to look in 1 and 2 and everything looks fine. Makes no sense. Any ideas?

Are the spark plugs on 1 & 2 wet or dry when you pull them out after the engine has run?

Put 4 new spark plugs in and restart the engine

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The plugs are dry... Thanks will do. The plugs are relatively new and were running fine but I will try that. The weird thing is that they are sparking.

Maybe a bit of a long shot, but I had a strange electrical issue with Jr’s 944 and it frustrated me for weeks ... was also getting spark and fuel. Turned out to be a hairline crack inside the distributor cap and the spark was jumping. It was only visible with a hard look and a macro lens. I’d replaced the rotor but thought the cap was fine .. 😔

Thanks so much for the tip I will look. Just so weird it ran perfectly before.

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Now ask yourself "How can that be so?"

Yes I suspected the Injectors were not firing but having said that, with a screwdriver to my ear I could swear they were clicking. 1 and 2 are off the same DME junction and as injectors earth to fire I might have to recheck that. I had 12v at the injector leads. More head scratching...

Have you tried swapping things around to see if the problem shifts. Only move one thing at a time.

I will do on the weekend when I can get to it next. I am also going to look for a break in the injector harness. If 1 and 2 are not firing they are off the same DME connection so I will check what I can of that too. Thanks for the help so far!

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Aren't the injectors on this model batch fire? ie all injectors get triggered together?  If so, it would be very difficult to hear if some aren't firing due to the noise and pressure pulsation in the fuel line from the other working ones. OZ930 suggestion has great merit.

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I'm not sure. The wiring diagram shows 1 and 2 being on one DME terminal and 3 and 4 being on another (14 and 15 I think) 1 and 2 are likely not firing. There is definitely spark at 1 and 2 plugs. I put a screwdriver on each injector and put my ear on it and they seemed to be clicking but yes with the lifter noise (even as muted as it is) it is hard to tell. I will have to get into it again on the weekend.

The '84 to '88 Carrera 3.2's were all batch fire even though they too also had two DME terminals like your car so I assume 944 of the same era are batch fire too. Consequently sound like your 1 and 2 injector harness may have a fault as it would be near impossible for two injectors to fail at once.

Thanks Peter, that's what I am thinking. All 4 injectors just came from 944 online. The wires on the harness are pretty hard. All 4 gave 12V but I didn't check the Earth side because it's so awkward getting a Multimeter to the DME plug then back to the harness but I will have to do that. I really appreciate your input.

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I wouldn't do that.  Since you have confirmed that all injectors are getting 12volts, just check that there is continuity between the earth contact in the inject harness plug leads on 1 or 2 and the body or engine block as this is easier to do.  If there is no earth continuity as you suspect I would then carefully check that harness in the engine bay for damage.  Just think back to when you were last using box cutters/snips/hammers/oxy gear/axes to "clean up dodgy factory wiring" . Victim of friendly fire perhaps? Good luck!

You changed the injectors right, they are new/unproven?  Entirely possible they're old stock sitting around a long time - clogged, jammed or otherwise nonfunctional.  I'd pop the rail back off and swap injectors 1&4 and see if the missing cylinder changes.

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Thanks Peter. Friendly fire is always a possibility although I was an aircraft engineer for a long time and I am pretty careful. It will be interesting to see what it turns out to be! Hi Steve, Ian and  the guys at 944online are pretty reliable. The injectors are sold checked as checked and flow tested but yes I will do as you suggest if I don't find another answer. Thanks for your help.

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At least it's a pretty quick and easy test to swap two over, and will certainly guide you in a direction.  Odd that it's 1&2, as they are on opposite phases of the crank, so in a modern scenario would be electrically separate, but others have pointed out the batch fire arrangement and you've traced them both to one pin on the DME, so that is certainly suspicious as you've said. One thing to note is that getting a spark in air is different to a cylinder under compression.  A friend of mine developed electronic ignition systems for bikes during the 90's.  His load test was to test the ignition system in air but snap the ground electrode off so the spark has to jump much further.  That was a way to somewhat simulate sparking under cylinder compression and fuel density etc and showed up weak coils or HT wiring issues.  That was with multi-coil systems so a bit different, but If the 944 has a single coil with dizzy cap / rotor etc those can also produce different failure modes that might do what you're seeing.  You'd have to satisfy yourself that it was safe for the DME though, I would think so but flag the possibility it might damage it.  Swap injectors first.

Thanks Steve, I definitely will swap injectors and tread carefully. First step will be checking Earth continuity in the injector loom. It's quite possible I broke a wire when tucking them back into the plastic cover. They are pretty hard from age and heat. Hopefully I will have a good result on Saturday when I get to it. The help and suggestions I have received here have been fantastic. Much appreciated.

You mentioned changing the reference sensors, have you checked the resistance of the replacements.  The reference sensor can cause this issue as well as the harness from them  to the dme.

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Thanks so much for the reply. They are new sensors from 944online. I certainly can check them but I can't imagine why it would only affect 2 injectors of 4. That really would be strange. I am going to go over everything on Saturday and I will post my results!

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Not strange, impossible! LOL

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If the reference sensor goes bad but the DME is still seeing the synchro signal and a week reference. The DME can still pick up a signal at slow crank speed but the signal disappears as rpm increases. You get fire up but the engine doesn’t see the reference signal the second time around so it never fires the next batch. The whole time your getting syncho so the DME is looking for the reference signal to fire again. The engine rpm drops until it picks up a reference signal again and fires the next batch, and this repeats hence its only running on 1/2 or 3/4 in your case. Just for quick confirmation an engine running on 2 cylinders will still rev, in this instance you won’t be able to rev it. If you multimeter the new sensors you can rule them out pretty quickly. It’s a good place to start, especially because you’ve replaced them recently. It’s a good habit to check them before install. I have also seen a bad relay still run the fuel pump but had a bad voltage drop and couldn’t idle the engine. The pump was running just enough to supply fuel to the rail and start up every now and then for a rough idle much like I mentioned above.

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Okay finally got to it. Checked continuity from DME to injector harness- all good swapped injectors 3 and 4  to 1 and 2 to positions put cup under all 1 and 2 fired. So it looks like I have 2 bad 'new' injectors. Always the simple things! Thanks to everyone who helped on this one. I should have checked earlier but confirmation bias led me to leave 'new' injector testing until last... Okay finally got to it. Checked continuity from DME to injector harness- both positive and negative - all good swapped injectors 3 and 4  to 1 and 2 positions put cup under all and 1 and 2 fired. So it looks like I have 2 bad 'new' injectors. Always the simple things! Thanks to everyone who helped on this one. I should have checked earlier but confirmation bias led me to leave 'new' injector testing until last...

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Good to hear. Have you tried to free them up, they can stick closed from new or when they haven’t been run for a long time.

I remembered watching this vid, so your problem rang a bell.  Used/cleaned injectors apparently don't like sitting around for long.  Excellent diagnosis work in this episode. Resolution was one stuck and two weak injectors, cleaned and flow tested as good but waiting 12 months for installation.  Great channel this one.

Ian at 944Online is sending 2 replacements DHL express so I should have them by the weekend or Monday.