M50NV cranks but doesn't fire after running fine for a long time

[LTDScott]

This is a continuation of this thread from 3 years ago where I first posted that my M50NV (3/92) swapped '87 325 Lemons racer abruptly stopped running on track after having successfully run this same combo for many racing miles. We did a bunch of trackside diagnostics but ultimately gave up, and I parked the car for 3 years. Now I'm trying to bring the car back to life and I can't remember all of the details of what I tested and checked previously, so I started the process of diagnosing the problem from scratch.

Here's the sequence of events.

About a week ago, I checked the resistance of the crank position sensor and it was 800+ ohms. It was at that time I discovered that the resistance spec for the crank sensor in the Bentley manual was wrong. The correct spec is ~540 ohms. I think that incorrect spec in the manual may have sent us chasing our tails at the track, but regardless it was clear that the sensor in the car was no good. I went to the junkyard and found a donor crank sensor that measured 530 ohms, installed it in the car, and it fired right up! Video proof:

https://www.facebook.com/ltdscott/vi...5232537320300/

Even though it ran, it was very sluggish to rev, and about 10 seconds after the video ended, the engine stopped running and refused to refire, no matter what I tried. I even swapped in a spare computer, but no go.

Fast forward to today, and I decided to methodically check as many components as possible. Here are all of the results.

-Battery voltage: 13V+ (it's on a battery tender)
-Fuel pressure to the rail: 40 PSI (I have a gauge plumbed in just before the fuel rail) and I can hear the pump priming.
-Ignition coils: 12V+ at pin 15 of each connector, and the resistance on each coil was 0.4 - 0.5 ohms. All within spec.
-Main relay: Confirmed 12V+ at pins 30 and 86. Confirmed ground at pin 85. Confirmed 12V+ at pin 87 with the relay installed. All within spec.
-Fuel pump relay: Confirmed 12V+ at pin 30 with ignition off. Confirmed 12V at pin 86 with ignition on. Confirmed ground at pin 85. All within spec.
-Crank position sensor: 533 ohms. Within spec. I also pulled back the rubber boot on the connector and saw a green band on the internal wires which confirms it is connected to the right sensor.
-Cam position sensor: 1380 ohms. The Bentley manual doesn't have a spec for this, but I did some searching online and other people stated ~1350 ohms so I can only assume this is good. That said, this was a new sensor as of 3 years ago, but unfortunately I can't find the old sensor that I pulled out to compare to. I also pulled back the rubber boot on the connector and saw a red band on the internal wires which confirms it is connected to the right sensor.
-Throttle position sensor: Confirmed 5V at pin 3. Resistance between pins 1 and 3 is 3970 ohms. Resistance between pins 1 and 2 going from idle to WOT was 1360 - 4500 ohms. All within spec.
-Engine compression: My compression tester apparently died, so I got a loaner from O'Reilly and oddly the readings were all 57-70 PSI (most were ~70, cyl #5 was 57). I was expecting 100+ PSI readings (as I got in the past), so I'm dubious about the accuracy of the loaner tester, but at least I confirmed the engine does have compression and the cylinders are relatively even. I'll buy a new tester and will try again.

I confirmed the injectors are firing by listening for clicking with a stethoscope, and I can even see the fuel spray on the pistons with the spark plugs out.

I confirmed there is spark by connecting every coil to a spark plug and then grounding the plug and coil. The spark looked strong with every coil. The spark plugs themselves look great and don't show evidence of fouling or lean mixture.

I also cleaned up the computer ground point (I'm using a factory bolt behind the passenger strut tower) and replaced a suspicious looking ground ring terminal for the coils and/or injectors on the front timing cover.

After all of this work, I put everything back together, and with the fuel pump relay disconnected, it was clear the engine wanted to fire. So I plugged the relay in and the car immediately fired up. It was idling kinda oddly, and I noticed that I had neglected to connect the MAF to throttle body elbow, so I pushed it onto the TB and the engine sounded great. I then shut it off to properly tighten the elbow, and just like last week, the engine refused to fire again!

It's been very frustrating to have the temporary glory of a running engine taken away immediately!

It seems I have fuel, air, spark, compression (at least some), so the only thing I can't fully check is timing. What am I missing here?

One thing to note is I don't have a check engine light wired up for a stomp test. It was previously recommended that hook one up to check the codes. But would I expect to see any codes if the engine isn't actually running?

Any help is greatly appreciated!

[Wanganstyle]

This is a continuation of this thread from 3 years ago where I first posted that my M50NV (3/92) swapped '87 325 Lemons racer abruptly stopped running on track after having successfully run this same combo for many racing miles. We did a bunch of trackside diagnostics but ultimately gave up, and I parked the car for 3 years. Now I'm trying to bring the car back to life and I can't remember all of the details of what I tested and checked previously, so I started the process of diagnosing the problem from scratch.

Here's the sequence of events.

About a week ago, I checked the resistance of the crank position sensor and it was 800+ ohms. It was at that time I discovered that the resistance spec for the crank sensor in the Bentley manual was wrong. The correct spec is ~540 ohms. I think that incorrect spec in the manual may have sent us chasing our tails at the track, but regardless it was clear that the sensor in the car was no good. I went to the junkyard and found a donor crank sensor that measured 530 ohms, installed it in the car, and it fired right up! Video proof:

https://www.facebook.com/ltdscott/videos/10215232537320300/

Even though it ran, it was very sluggish to rev, and about 10 seconds after the video ended, the engine stopped running and refused to refire, no matter what I tried. I even swapped in a spare computer, but no go.

Fast forward to today, and I decided to methodically check as many components as possible. Here are all of the results.

-Battery voltage: 13V+ (it's on a battery tender)
-Fuel pressure to the rail: 40 PSI (I have a gauge plumbed in just before the fuel rail) and I can hear the pump priming.
-Ignition coils: 12V+ at pin 15 of each connector, and the resistance on each coil was 0.4 - 0.5 ohms. All within spec.
-Main relay: Confirmed 12V+ at pins 30 and 86. Confirmed ground at pin 85. Confirmed 12V+ at pin 87 with the relay installed. All within spec.
-Fuel pump relay: Confirmed 12V+ at pin 30 with ignition off. Confirmed 12V at pin 86 with ignition on. Confirmed ground at pin 85. All within spec.
-Crank position sensor: 533 ohms. Within spec. I also pulled back the rubber boot on the connector and saw a green band on the internal wires which confirms it is connected to the right sensor.
-Cam position sensor: 1380 ohms. The Bentley manual doesn't have a spec for this, but I did some searching online and other people stated ~1350 ohms so I can only assume this is good. That said, this was a new sensor as of 3 years ago, but unfortunately I can't find the old sensor that I pulled out to compare to. I also pulled back the rubber boot on the connector and saw a red band on the internal wires which confirms it is connected to the right sensor.
-Throttle position sensor: Confirmed 5V at pin 3. Resistance between pins 1 and 3 is 3970 ohms. Resistance between pins 1 and 2 going from idle to WOT was 1360 - 4500 ohms. All within spec.
-Engine compression: My compression tester apparently died, so I got a loaner from O'Reilly and oddly the readings were all 57-70 PSI (most were ~70, cyl #5 was 57). I was expecting 100+ PSI readings (as I got in the past), so I'm dubious about the accuracy of the loaner tester, but at least I confirmed the engine does have compression and the cylinders are relatively even. I'll buy a new tester and will try again.

I confirmed the injectors are firing by listening for clicking with a stethoscope, and I can even see the fuel spray on the pistons with the spark plugs out.

I confirmed there is spark by connecting every coil to a spark plug and then grounding the plug and coil. The spark looked strong with every coil. The spark plugs themselves look great and don't show evidence of fouling or lean mixture.

I also cleaned up the computer ground point (I'm using a factory bolt behind the passenger strut tower) and replaced a suspicious looking ground ring terminal for the coils and/or injectors on the front timing cover.

After all of this work, I put everything back together, and with the fuel pump relay disconnected, it was clear the engine wanted to fire. So I plugged the relay in and the car immediately fired up. It was idling kinda oddly, and I noticed that I had neglected to connect the MAF to throttle body elbow, so I pushed it onto the TB and the engine sounded great. I then shut it off to properly tighten the elbow, and just like last week, the engine refused to fire again!

It's been very frustrating to have the temporary glory of a running engine taken away immediately!

It seems I have fuel, air, spark, compression (at least some), so the only thing I can't fully check is timing. What am I missing here?

One thing to note is I don't have a check engine light wired up for a stomp test. It was previously recommended that hook one up to check the codes. But would I expect to see any codes if the engine isn't actually running?

Any help is greatly appreciated!

Leak it down.

Perhaps engine just kicked the can


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[LTDScott]

I've got a new compression tester coming in tomorrow. Will give it another shot.

[LTDScott]

Tested compression again (all spark plugs out, fuel pump relay unplugged, throttle body wide open) with a new tester and the results are the same as the last test:

1: 75
2: 75
3: 68
4: 75
5: 65
6: 78

Per Bentley they should be 142-156 psi, so something is definitely up with this engine. I'll do a leakdown test on it later this week, but in the meantime, what could cause ALL of the cylinders to lose half of their compression?

[e30austin]

My best guess is, you have washed the cylinder walls, due to cranking it so many times and fuel not being burnt off, from it not firing.

Pull the plugs out, pull the main relay, crank the engine for 10 seconds and ensure all the fuel is out of the chamber, drop a couple table spoons of oil down each cylinder, reinstall plugs and repeat. My guess is compression is so low, that you can't get the engine to fire. Now, once it fires, keep your foot on the throttle for a few minutes. You'll need to change the oil a couple times, as it's going to smoke like mad after you get it running. Your oil is probably a good percentage fuel, at this point.

If that doesn't do it, something I've seen, is the woodruff key, that holds the front crank pulley in place, sometimes shears off and then will give you false readings, since the front balancer is out of time from the engine. Make sure you can't move the pulley back and forth. Seen it happen on a couple track cars that see high RPMs and a fair amount of abuse.

Best of luck!!!

[AndrewBird]

Are you trying to run it on 3 year old gas? If so, drain the gas tank and get some fresh gas.

[LTDScott]

Are you trying to run it on 3 year old gas? If so, drain the gas tank and get some fresh gas.

Excellent question. No, I completely drained the tank when I parked the car 3 years ago and put a few gallons of fresh gas in.

Now for today's update.

Yesterday I first checked the oil (shoulda done that a while ago) and it definitely smells like gas and seems to be thinner than it should be, so that tells me I have been getting fuel washdown. Then I performed a leak test. I first found TDC on the crank wheel and started testing with cylinder 1, then moved along the firing order. I found TDC on the other cylinder by putting a long zip tie in the spark plug hole and watching it until it stopped moving. Here are the results I got:

1. 96%
2. 8%
3. 64%
4. 64%
5. 15%
6. 94%

It dawned on me that if I wasn't at actual TDC with both of the valves closed, it could give false readings, so I did it again, and this time if the reading was low I slowly turned the crank to see if I got better results. I did:

1. 96%
2. 92%
3. 93%
4. 87%
5. 89%
6. 94%

I noticed on a couple of the cylinders that as I was applying pressure with the leak tester, the pressure gauge on the cylinder side would go up slowly (much slower than the air source from my compressor) and then suddenly spike up, like something suddenly closed the leak. Maybe the rings suddenly sealed? I dunno. What little leakage I could detect was definitely coming out of the oil filler, which indicates rings.

I had to stop for the day, but before I did I poured a little 40W oil into each of the cylinders to see if it would help seal up the rings at all.

Today I decided to compression test the engine again to see if the oil I put in last night helped. Nope, I tested cylinders 1-3 and they were in the 60 psi range, so I just stopped there. WTF? Decent leakdown results but low compression?

At that point I decided to just crank the engine for about a full minute with the fuel pump relay unplugged to clear the cylinder, then put the relay back in, and the engine fired right up. It sounded and acted great, then suddenly wasn't happy - throttle response while revving was pretty lethargic and it stalled. I was able to get the engine to start and run a few more times after that, and had the same experience - alternating between running well and running poorly. Finally my battery started to get weak from cranking, so I hooked it up to a charger and called it a night.

I'll keep working on it over the weekend. I'd like to do another compression and leakdown test now that the engine has been running, and I still want to pull the valve cover and crank pulley to see if cam timing is correct. Even if the engine isn't running consistently well, I still want to figure out how I could have horrible compression across all cylinders but decent leakdown results.

[pazi88]

If your engine starts and dies immediately, the lifters can get overfilled, leading valve that is left open just little bit. Cranking engine long time fixes the problem.

The problem sounds like faulty and intermittent crank sensor.

[e30austin]

I would say that you need to let the engine run/idle for at least 5-10 minutes before trying to rev it up. Give it ample time to build compression again. If it does the same thing again, after trying that, then I will agree that it's got a faulty crank position sensor.

[Wanganstyle]

M50NV cranks but doesn't fire after running fine for a long time

I would say that you need to let the engine run/idle for at least 5-10 minutes before trying to rev it up. Give it ample time to build compression again. If it does the same thing again, after trying that, then I will agree that it's got a faulty crank position sensor.

Or just grab spare crank sensor from friend or parts bin; someone in lemons world or local e30 person must have one loose

[e30austin]

Or just grab spare crank sensor from friend or parts bin; someone in lemons world or local e30 person must have one loose

This is true. I always keep spare, known good sensors on hand.

[Wanganstyle]

M50NV cranks but doesn't fire after running fine for a long time

This is true. I always keep spare, known good sensors on hand.

Wish I had a running m50nv and was local; would love to help the homer run again

[LTDScott]

I think I found the smoking gun:



The secondary timing chain tensioner broke and the intake cam is off by about 45 degrees.

The plastic guide on the top of the tensioner is gone, and I was able to fish some pieces out of the head. The chain is currently metal to metal.







I'm surprised it ever ran like this at all, but it sure does explain the crappy compression numbers but decent leakdown results. That said, the fact that I'm able to get 90%+ leakdown results tells me the pistons and valves probably didn't meet.

All along I was basically intending to yank this engine and replace it with a fresher or lower mileage one (as I mentioned previously, I think it may have a blown head gasket and it has 250K+ miles), but I didn't want to do that until I figured out whatever was causing the current engine to not run or run properly. I was concerned that the root cause was in the electronics somewhere (harness, computer, etc.) and just swapping in a new engine wouldn't solve anything.

For that reason, I think I might buy a new tensioner (thankfully aftermarket ones are only $40), correct the cam timing, throw everything back together, and hopefully the engine will start and run properly. If that's the case, then at least I'll feel confident that all of the components surrounding the engine are in good working order, and then I can think about potentially swapping the engine.

[ZekeTheSneak]

Glad you figured it out! I was going to say sometimes the obd1 ecus fail internally and leave a single injector open. Happened to me and washed a cylinder.

[LTDScott]

The crank position sensor I pulled off the engine tested bad, and all of the cranking has definitely washed down the cylinders with gas based on the smell of the oil, so those things may have contributed to the story, but clearly the cam timing is the biggest issue I need to address first.