91 HO Shut Off While Driving

[AdroitKirby]

H0rnbrod talking about ignition coil failure 

 

I might also have this problem. Long crank time and intermittent engine shut offs under load

[Eagle]

On 6/3/2021 at 3:22 PM, jdog said:

Take it out again, you won't be able to figure out what's failing unless you get it to fail, once it has failed check the coil for heat and see if there is spark. My money is on the coil.

 

My vote is the CPS.

 

Could be either one.

[omega_rugal]

carry a spray can with water, if it happens again, spray the CPS to cool it, if it starts, it´s the CPS, also when a Coil fail is because gets hot INSIDE, not outside, external heat can gradually enter the core of the coil, hot core equals no spark, and it takes a while to cool down ex overnight, just to be safe, relocate the coil away from the engine block and make sure is properly grounded, a coil with bad ground tends to overheat internally...

[acfortier]

My experience was a bit different than @kryptronic's. I was able to start it again after letting it sit for about 10 minutes or so. Not sure if it could theoretically cool off that fast?

 

Also, the more I thought about it, I'm assuming there isn't much wiring in the cab that would relate to starting the vehicle. If it cranks, I'm assuming that's the extent of the wiring that would be cab-side?

[kryptronic]

8 hours ago, acfortier said:

My experience was a bit different than @kryptronic's. I was able to start it again after letting it sit for about 10 minutes or so. Not sure if it could theoretically cool off that fast?

 

Also, the more I thought about it, I'm assuming there isn't much wiring in the cab that would relate to starting the vehicle. If it cranks, I'm assuming that's the extent of the wiring that would be cab-side?

 

Page 1 of this thread lists multiple possible culprits which have all been checked to some degree.  I only saw it shut off once, and that was at operating temp.  You saw it once, again at operating temp.  On HO vehicles, until operating temp is reached (~210 degrees) the computer is off and the mode is known as open loop.  The computer controls nothing (virtually) until you hit 210 degrees.  From that point, the computer kicks on, the system goes into closed loop, and based on input from sensors, adjusts idle.  This would point to an electronic/sensor issue, or bad fuel based on the data.   We're making the assumption here the computer is receiving some kind of bad data from something and choosing to shut off for protection.

 

A bad crankshaft position sensor (CPS) would cause a no start condition from the point of failure, and they don't come back to life.  I've checked the CPS recently.  Rule out the CPS.  Also, only replace it as a last resort.  It's probably the most difficult sensor to install.

 

A bad camshaft position sensor (CMS) would cause rough idle, not a shutdown.  Plus I replaced the distributor last year, which contains the CMS, and the CMS is new.  Rule out the CMS.

 

A bad throttle position sensor (TPS), idle air control valve (IAC), mass airflow pressure sensor (MAP), or bad engine coolant temp sensor (ECT) would cause rough idle, not a shutdown.  All of them have been checked within the past year.  Rule out those sensors.

 

I've been over the fuel system and have replaced the pump, cleaned the lines and installed a new filter.  When I ran into the shutoff/no start condition, there was plenty of fuel in the fuel rail - indicating everything is fine there.

 

The only suggestion I could not rule out was an overheated ignition coil.  Running the vehicle for hours on end during the hottest days this summer and even getting the aux fan to run did not result in any problems.  So maybe it's the coil, maybe not.  Maybe it's just the three year old gas in the tank.

 

To answer your question, I don't think there's much wiring at all in the cab that would affect this.  Maybe some ignition wiring.

[Eagle]

3 hours ago, kryptronic said:

 

A bad crankshaft position sensor (CPS) would cause a no start condition from the point of failure, and they don't come back to life.  I've checked the CPS recently.  Rule out the CPS.  Also, only replace it as a last resort.  It's probably the most difficult sensor to install.

 

 

I disagree. The CPS can fail due to heat soaking and, once it cools down, it does come back to life (until it heats up again).

[kryptronic]

46 minutes ago, Eagle said:

 

I disagree. The CPS can fail due to heat soaking and, once it cools down, it does come back to life (until it heats up again).

 

Thank you.  I stand corrected.

[zomeizter]

Take your timing light with you and a can of carb cleaner next time you go for a ride, when it shuts off, hook your timing light up to the coil wire and crank the engine to check for spark. If you have spark but the engine still won't start, spray some carb cleaner into the intake manifold and crank again, if it starts, you have a fuel delivery problem...

If the spark check failed prior to spraying into the intake, then proceed to check CPS, coil and wiring as previously recommended, good luck.

[kryptronic]

I'm upset that the truck has done this to you.  As such, it's has been on my mind as much as when I experienced it earlier this year.  One bit of info that I didn't have before was from @Eagle, who can always be relied on for accurate info, that the CPS could experience a heat soaking issue and become inoperable for a while, then start working again when cooled off. 

 

Based on my experience with the issue, I had the truck fail on the road after moderate driving around my local area.  During that drive, I was enjoying the truck, and between 2nd and 4th gears quite a bit.  When driving a vehicle with a manual transmission, I don't use the brakes very often.  Usually just to finish a stop.  My driving style has me shifting gears up and down often, usually between 2800-3200 RPM.  I had the issue on the road.  Subsequently, I tried to replicate while parked, and couldn't.  I had the vehicle under extreme heat conditions for several hours while parked to try to replicate the issue, and it never happened again.

 

Knowing the CPS sits on top of the bellhousing close to the transmission, and knowing now that it can experience a heat sink issue, and also knowing that the failure only ever occurred while actually driving the vehicle, I'm thinking the transmission could be producing excess heat.  Now that I've been thinking about it, I recall making a mental note to check the clutch fluid level.  Over the course of the time I had the truck, I recall the feel of the clutch pedal degrading very slightly over time.  Maybe.  Or maybe I was becoming more sensitive to it.  Regardless, with that in my mind, and knowing I never changed the transmission fluid, I've got the theory the transmission is running hot and causing the CPS to fail due to heat soak.  If this is the case, then replacing the CPS will not fix the issue - the CPS is not bad, the transmission is hot.

 

So, based on that theory, I'd recommend you drive the truck, and get on the transmission.  Try to replicate the issue on the road.  If and when it fails, check the transmission to see if you think it's excessively hot.  You'll just need to wait for things to cool off to get home, but that should take under an hour.  Then, change the transmission fluid.  It's an AX-15, so use Redline MT-90.  Then check the clutch master cylinder for fluid level, and fill with DOT3 or better brake fluid, as required.  Then repeat the testing procedure.  I think these are two items that you need to do anyway, so the testing procedure ahead of time is simply to give you a baseline as to whether or not the fluid checks/fills helped. 

 

As for the transmission, I recommend you drain and fill it with all new fluid.  There's no telling what might be in there, and it's almost certainly not Redline MT-90.  Probably motor oil, but it could be ATF or whatever the last shop/person who changed it had on hand.  I recommend Redline MT-90 based on hours upon hours of research on the subject specific to the AX-15, and my own personal experience using it with other AX-15 equipped Jeeps I've owned.  There is none better.

 

That's my input at this time.  I welcome thoughts on the above from other members.  Good luck, @acfortier.

 

 

 

 

[acfortier]

Interesting, never would have thought that the transmission could be running hot, causing a heat soak. It makes sense in my mind—since it hasn’t been done, I’ll add it to the list.

 

[omega_rugal]

On 8/27/2021 at 6:36 PM, Eagle said:

 

I disagree. The CPS can fail due to heat soaking and, once it cools down, it does come back to life (until it heats up again).

 

its basicly a magnet surrounded by a coil, when it gets hot, it loses its powers...

[omega_rugal]

How hot has anyone seen a MANUAL transmission get?

[ghetdjc320]

5 hours ago, omega_rugal said:

How hot has anyone seen a MANUAL transmission get?

That’s the thing, they don’t normally get very hot. Thus no factory provision for a cooler like an automatic. CPS could be an issue but it does seem like the HO CPS were less finicky with heat than the Renix era ones. I have been having very similar issues on my HO and have narrowed it down to the cam sensor and/or coil. Both of these get plenty hot as well and can display similar symptoms. CPS handles ignition timing, cam sensor handles fuel timing. The HO will not run without the cam sensor. But all 3 of those items can behave in a similar fashion. 

[omega_rugal]

oh that may be the problem, if the cam sensor stops working and the HO doesnt work without it it explains the shutdown...

[ghetdjc320]

39 minutes ago, omega_rugal said:

oh that may be the problem, if the cam sensor stops working and the HO doesnt work without it it explains the shutdown...

Yeah I thought it may keep running without it especially in closed loop but it’s shuts off immediately. Something changed with the cam sensor in the 94 model also. I switched to a 95 dizzy with cam sensor and it plugs right in. 

[acfortier]

I think I can probably rule out a cam position sensor—the entire distributor was replaced recently by @kryptronic

 

[ghetdjc320]

6 minutes ago, acfortier said:

I think I can probably rule out a cam position sensor—the entire distributor was replaced recently by @kryptronic

 

Coil? Cap, rotor plug wires? Just brainstorming here. It’s always the simple stuff it seems

[kryptronic]

12 hours ago, ghetdjc320 said:

Coil? Cap, rotor plug wires? Just brainstorming here. It’s always the simple stuff it seems

 

Coil is suspect due to age (~140k).  Entire distributor was put on (new cap, rotor, cam position sensor) was installed within the last 100 miles.  Plug wires are really nice and were installed within the last 1000 miles.

[omega_rugal]

10 hours ago, kryptronic said:

 

Coil is suspect due to age (~140k).  Entire distributor was put on (new cap, rotor, cam position sensor) was installed within the last 100 miles.  Plug wires are really nice and were installed within the last 1000 miles.

 

being new doesnt mean it won't get hot...

[EvilMJ]

I wouldn't trust any white/box aftermarket distributor now a days... CPS may still be your culprit.

[EvilMJ]

Meant CMS, but I like the input on the CPS too, it may be it.  And as I said long ago, coils go bad at end of life, and 140k is EOL.

[ghetdjc320]

Use the 94+ distributor on that. Much better cam sensor and is a straight bolt in. Make sure your indexing is perfect as well. You can see my recent post with a similar issue and the coil and indexing were to blame.

[Minuit]

2 hours ago, ghetdjc320 said:

Use the 94+ distributor on that. Much better cam sensor and is a straight bolt in. Make sure your indexing is perfect as well. You can see my recent post with a similar issue and the coil and indexing were to blame.

[Tex06]

On 8/28/2021 at 7:20 PM, omega_rugal said:

How hot has anyone seen a MANUAL transmission get?

Probably better in another thread but I saw case temps above 140F when I was running motor oil in mine. Redline dropped it to around 118F (back to back weeks in New Orleans summertime heat, same commute to work each day).


As for the OP's original problem, this sounds very similar to what I had happen except my '92 would cut in and out sometimes while driving (causing the whole truck to buck) or sometimes would go completely dead as he described. Turned out to be a bad solder joint in the main harness that was the common 5VDC for the TPS and MAF. When the engine was up to temp and the computer would go into closed loop it would lose signal from both sensor simultaneously (if I unplugged one, it would actually run better b/c the solder connection was high enough resistance that it could supply both sensors, but was enough of a connection that it could supply just one).  I ended up using a power probe to check the voltage at both while they were hooked up and was only seeing 2.7VDC when it should have been 5V. 

Once I figured that out, it was a 20 minute fix to open up the wiring harness and resolder the connection. I've got a writeup on it somewhere, this was 4 or 5 years ago at this point. 

[omega_rugal]

2 hours ago, Tex06 said:

Probably better in another thread but I saw case temps above 140F when I was running motor oil in mine. Redline dropped it to around 118F (back to back weeks in New Orleans summertime heat, same commute to work each day).

 

we probably should start putting coolers to manuals transmisions too.. and differentials can get hot under load...