Weird random dead spot with RPM job, but engine doesn't die

[dkmcgowan]

So I just did a bunch of work on the 89 Comanche...  It was running and driving without issue.  I rebuilt the head, cleaned the engine bay, replaced the distributor, converted from AW4 auto to AX15 manual, upgraded to newer style brake booster...

 

It ran and drove great...  I did still have an idle issue, so I also threw in a random 90 auto ECU and the idle issue was resolved.  I didn't do many test drives after the ECU switch, but did idle it awhile.

 

So I have took 2 drive with it, each about 30 minutes, all worked great.

 

I then took it out to a friends shop, we removed the master cylinder and brake booster again to remove the spacer to raise my brake pedal height, hooked everything back up, idled and ran fine, done...

 

Then when driving home I was in 5th gear and all of a sudden it was a complete dead spot as if engine was doing nothing but still running, then it was back, then again, it's only like 1 second.  I down shifted, it was still there, then it went away.

 

It came back 2 more times randomly on the way home, one time I pulled over and let it idle, it idled fine.  The next time I was coming to a stop light, it was idling fine.  I started to pay attention more and when it happens, my RPM needle drops to zero and instantly goes back to where it was, so the RPM's are gone completely.

 

It then stopped and didn't do it again.

 

I took the washer reservoir back and my overflow tank I had added near air box and checked all the wires.  This is just because it was the area I was working in.  I removed the one relay on that area and cleaned it up and made sure it was seated correctly.  I shifted some wires around to ensure they were not under tension.

 

I had my two fuel pump wires spadded together, so I also crimped nice new connectors on them and installed my ballast resistor again.

 

I took two more drives tonight and all worked fine.

 

But I'm worried and don't trust the car right now.

 

What would cause that behavior, a random dead spot and complete RPM drop, but recovering within 1 second and keep going, and the engine not dying?

 

I just ordered another ECU that is for a manual, I know my old ECU was causing idle issues and maybe the new ECU caused it to do this?

 

My guess would either be complete electrical failure somewhere, so maybe related to the new distributor installed, fuel issue (there wasn't one before, new fuel pump, new lines, new rail, new regulator, new filter), or the ECU.

 

Thoughts?

[86FUBAR]

Hum? You say the RPMS drop but the engine doesn't die , but it only does it when driving correct? Is there a loss in power ? Does this coencied with anything like a bumb in the road or ....? Does it feel like you shut the key off for a second then back on real quick ?

[dkmcgowan]

So far only when driving. There is a complete loss in power, you are acclersting and then all of a sudden nothing for 1 second, rpm drop, then all back to normal all in 1 second.

 

It's possible it was bumps, I could try to find some bumpy road to test this further.

 

It could be like the key was off for a second, but it doesn't die. It was night time and all lights and everything stayed on.

[Crash]

Consider ign, CKP, Fuel pump ballast resistor. 

Does engaging the clutch affect anything? Maybe bleed and check your MC/slave.

[dkmcgowan]

It could be ign, I did have to change some stuff to swap from auto to manual... when it happened I had ballast resistor removed and wires together with a piece of metal, I've since crimped new connectors and reinstalled ballast resistor. So far it hasn't happened again.

 

I will try the clutch if it happens again, at first I thought it was a transmission problem, why I can get gears to see what I would do, it switched gears when I down shifted and then did it again in the next gear.

 

How would clutch problem drop rpm quickly and let them go back up?

[Eagle]

I'd say it has to be ignition. The tachometer is electronic, not mechanical. If the engine stops firing but the vehicle is in gear and rolling, the engine will still be turning and a mechanical tach would still be showing RPMs. The electronic tach actually has no idea what the engine is doing, it's just counting ignition pulses. So if the tach drops to zero -- the ignition isn't firing momentarily. I would suggest focusing on that.

[schardein]

I have seen those symptoms from failing Crankshaft Positioning Sensors and Camshaft Positioning Sensors.  Although more commonly the vehicle will completely die and not restart for a 15-30 minutes, with the restart time getting longer every time it happens.  As corny as it sounds, disconnecting the crankshaft positioning sensor connecter (near the rear of the intake manifold) and then connecting it back again will often get an instant restart.

 

I've seen similar symptoms from a failing electric fuel pump, but not on Jeeps.  My Chevy did that, although only a couple times before failing completely a few minutes later.  Crawling underneath and banging on the fuel tank got a restart and a couple more miles down the road.  Again, sounds corny but worked.  

[Eagle]

A failing fuel pump would not interrupt the spark, and therefore would not cause the tachometer to drop to zero while the engine is turning over in gear at highway speed.

 

In this case, I think it has to be ignition related. CPS would be a good place to start.

[dkmcgowan]

Thanks for the suggestions. So I went to check cps sensor. When I installed ax15, it wasn't clipped on head like before, it was just hanging there coming up closer to brake booster. The line was melted in one spot. So my theory is when I pulled brake booster out and pushed master cylinder towards intake to remove spacer, I must have shifted the cps wire and I think it hit the exhaust manifold and burned a little, probably shorted out temporarily.

 

So the story all fits. I'm heading to dealer to pick up new oem cps sensor.

[87MJTIM]

My vote is the CPS.

 

My experience was this:  Back in the early 90s (92?), I would have a complete power loss - engine died, all power shuts off.  Then just as suddenly as I lost all power, the power would come back on.  It did this several times to me.  Then one night while driving home on the Washington beltway - in heavy traffic - it died and would not restart.  It was towed to a service station for repair.  The mechanic finally figured out it was the CPS.  He replaced it and all was fine, until....

 

A few months later, I was on my way to work when I had a complete loss of power again.  This time I had it towed to a Jeep dealership.  The mechanic listed to my description, looked under the hood.  He pulled up a wire that had melted through!

 

The first mechanic did not route the wire away from the exhaust pipes.  The wire just melted away.  

 

Now, when ever I do trans work or work at the back of the engine, I make sure the wire routes behind the cyl head to the passenger side, then loops back to the top of the trans.

[dkmcgowan]

So I believe we found the culprit for sure.  I took the old CPS off today and look and see.  I installed a new one and routed it over to the passenger side like it's supposed to.  When I rebuilt the head and replaced the transmission I lost the little clip things, so I used some zip ties and I think I got it out of the way of the exhaust manifold.