I finally got around to digging into my '75 350SE this morning and I found that it's only running on 4 cylinders... The two rear cylinders on the left bank and the middle and rear cylinders on the right bank (#3, 4, 6, 8) are dead. I believe this is a D-Jet issue since I've already been through most of the ignition system. I did some poking around with my multimeter and found the following:
- All 8 injectors test between 2.5 and 2.8 ohms
- The ground circuit on all 8 injectors tests 0 ohms to the negative terminal on the battery
- The injector drive wire from the ECM (pin #7) tests 3.0-3.1 ohms (which I find very odd; that gauge of wire at that length should have resistance of near-as-makes-no-difference 0 ohms) to 7 injectors; pin 7 to the + terminal at the injector for cylinder 4 is only 0.6 ohms
And now I'm stuck. I'm not sure if I should be looking for a wiring harness, or if I should pull the ECM and try to rework it (reflow the solder joints, replace the caps and resistors) or just what. I'm also confused as to why there's only one injector drive wire coming off of the ECM, because I was under the impression that D-jet fired in banks of 4, not all 8 at once. What should I be looking at next? Someone suggested using an analog voltmeter (because they respond more quickly) to see if the injectors on 3, 4, 6, and 8 are being fired; I was considering doing that, or grabbing my oscilloscope, or just pulling the fuel rail, but I'm not sure where to go from here.
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Some additional history that may be helpful: I bought this car in early 2009; when I bought it, it had fuel leaks from the lines on top of the intake manifold, and it would often run on 4 or 6 cylinders instead of 8. It seemed to be pretty intermittent, but was more prevalent when it was cold. A few days after I bought it I managed to overrev the snot out of it (no tach, but based on the correct euro speedometer i recently bought and some calculation i'd say i hit something comfortably north of 7,000rpms) by accident. It did not end up doing any permanent damage to the engine, but I took it in to a shop that I used to be a service adviser at to have it checked over and have the intermittent miss looked at.
The shop found (and I went by and confirmed) that one of the timing chain guides was broken, the chain was stretched and the distributor was worn out. I had all of the guides and the chain replaced, the distributor was replaced and the valves were adjusted according to the specs on the sticker on the valve cover. Compression was checked and found to be 175# in all 8 cylinders. When I picked it up it was running consistently on 4 cylinders; it has not run on 8 since then. I parked it inside with half a tank of gas (bad, I know) and no fuel stabilizer (even worse) and it's sat from June of 2009 until now. It started right up today after the battery was charged, despite only running on 4 cylinders, but I'd like to get it running on all 8 so I can at least get the tank of gas that's in it run through and get some stuff that's not a year old in it.
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So yeah, any help would be appreciated. Thanks guys.