84' 528e Heavy Misfiring + Wont Idle - Help please!

[Mapletree]

I'm a bit lost on what the issue is with my car. I was driving home from work and it just sorta sputtered out and died on the highway. I was able to limp it home, and it will stay alive if you give it throttle, but it misfires heavily the whole time and dies the second you let off.

As far as I can remember, the ICV has never been plugged into anything. I believe the PO bypassed it in some way, and I never cared to change it because it ran just fine for many years, but it’s possible this could be good info. The distributor was replaced right before Thanksgiving for a trip I took, as was the timing belt. The coil pack was replaced about 3 years ago. The crank speed sensor on the transmission is really old and busted-looking, so I’ve got one on the way but I don’t really know how to test it without just swapping it out. I’ve got an intake boot and a valve cover breather hose on the way as well, but I don’t see any gaping holes that would make me think it’s a vacuum issue. Still, I’m not going to rule it out with how old some of these lines are.

So far, I’ve gone through and checked the following:

• Checked for spark: Spark light is getting good strong flashes. However, I’ve got an order on the way from FCP with plugs and wires, as the current ones are getting pretty old. I'm not confident this will fix the issue.
• Fuel pressure: I'm getting 40–44 psi while it’s running. When shut off, it slowly sinks back down. After about 3 minutes, it was at 20 psi.
• Checked injectors: Did the old screwdriver check, and they all sound like they’re ticking.
• Pulled plugs: They were all wet with gas.
• Cold start injector: I completely disconnected the cold start injector on the intake manifold to see if it was possibly stuck open and dumping fuel—no change.
• Grounds: Made a temporary ground with jumper cables from the engine to chassis and negative battery to chassis—no change.
• TPS: Checked the TPS; it does not seem to be working and is caked with old and new oil. I’ve got a new one on the way, but I’m not sure how much this could play a role or how long it’s been non-functioning.
• AFM: Checked the AFM via the pins, as I couldn’t open the black plastic due to some strong silicone used to reseal it previously. The temp sensor was within spec. I tested the resistance while slowly opening the flap (off the car), and it seemed to climb gradually as it should, with some dead spots. However, I don’t think the dead spots are the issue, as it sputters regardless of RPM or throttle. I then checked the voltage while the car was on and was getting ~7 volts while fully open. The battery was pretty dead, though, so that number is probably off. I checked the voltage coming from the ECU on the connector and am getting 10.25 volts. I swapped in a spare ECU and got the same number. What is the proper value here?
• Main relay: Swapped it out just to be safe.

Any insight, next steps, suggestions, etc. is very much appreciated. I am not as well-versed with the Motronic system, as for much of the time I’ve owned it, I would just bring it to the shop if the problem wasn’t obvious to me but I’d like to try to do this on my own.

[turbodan]

The coolant temp sensor has a huge effect on fueling.  I would check the sensor, connection and wiring.

Sounds like it's gone pig rich, one way or another.  The coolant temp sensor is a negative temperature coefficient thermistor, as temperature increases the resistance drops.  If the DME is seeing high resistance or an open circuit it will assume the engine is Siberian cold and fuel accordingly.

[Mike W.]

The thing that jumps out at me is the wet plugs and fuel pressure dropping off so quickly. Dirty injectors can leak, but not that quickly and wouldn't cause wet plugs. Turbodan's suggestion is a good one, I'd also look at the Fuel pressure regulator. Pull the vacuum line off it, start it up and see if fuel comes out the vacuum port on it. I'm not sure if the fuel pump has enough capacity to both maintain fuel pressure and pump a bunch of extra into the manifold, but it probably does.

No the ICV, it has no effect on mixture, unplug the TPS to eliminate it, but even if the WOT switch was shorted I doubt it would make it run that badly.

[shizzel669]

Definitly check the coolant temp sensor as turbodan said.
One time I had a similar problem on my 635csi. Turns out it was a broken resistor which was soldered under the boot of the sensor connector. Took a long time to figuring this out. I think there was a recall for m30 engines back then where they soldered a resistor on to the cts, because some engines had a bad idle. Don't know if m20 engines had this recall too, but worth checking for it.

[Mapletree]

ok so maybe a dumb question, but which temp sensor should i be looking at. there are four on the thermostat housing that I see. is it possible to try jumping the connector to see if its bad or does it need a super accurate reading?

also is it a good test to swap the two reference sensor connectors going to the bell housing? I'm thinking if the speed sensor is bad, and i swap it to act as the position sensor, then if i get a no start it could be what's bad. would I need to swap the physical location of the two sensors in the bell housing, or does it not matter?

I forgot to mention that the car has had a single in-tank walbro pump put in by the PO. will be checking the fpr shortly. as for leaky injectors, i wouldn't expect them to all go bad all at once unless some other factor caused it

[Mapletree]

Update:

looked in the Bentley a little bit and figured out which sensor to measure (furthest back toward intake manifold) its about 55f out today and it measured at 2043 ohms which seems right i think. its in the sun and the Bentley says 2100-2900 at 68f. should i test further? maybe a dumb question but how do i check the connection/wiring?

also i realize now that the reference sensor location does matter . measured both of them by the Bentley and the seemed good - 950ish ohms across 1&2 on both and open on 2&3 on both.

pulled the vacuum tube after running it for a second and it was completely dry. couldn't do it while running as it dies and I'm by myself.

[shizzel669]

The CTS for the DME should have a blue colour. Just pull up the rubber boot of the connector, then you can access the wiring or eventually the mighty resistor.
Otherwise you could measure ohms directly at the DME connector. The same ohms should be measured there as on the CTS.

[Mapletree]

Welp, it turns out what happened was, the head stud head snapped off and lodged itself between the camshaft lobe and the valve. The piston and valve then smashed into each other thus pushing into the camshaft and snapping it. Somehow it was able to start still and was overcompensating with the gas. I guess it's rebuild or swap time.