[DrDan68108]

My 1995 Dakota has run almost trouble free... but recently has gotten a weird problem.

The truck has about 180k miles on it and deferred maintenance (insert face-palm graphic here) had caught up with it: driveability problems (rough idle, stalling, the usual stuff) plus "throwing" a trouble code. That code led me to the EGR valve. Replacing that helped a lot and haven't had the check-engine light come on since. New wires and plugs (replaced the platinum with OEM-spec Champs, per advice here on Allpar.) Spray cleaned the throttle body (gunk city but not as bad as the old EGR). So all this has helped, except...

If the engine is warm, and I shut it off for a few minutes (like when filling the gas tank, for example), the engine will crank strongly but won't fire up. How have I been getting it to start again? Nothing...for two-plus hours, when it'll start like nothing had happened before. (The engine will typically restart only if I try right after shutting it off when warm, or always when cold.) No problems when the temperature gauge reads below the normal range, except when I have to wait until it's stone-cold because I shut it off when warm.

Until I did a lot of testing until I could re-create the problem, it was as if there were a Murphy's Law sensor under the hood that could detect, for example, being in a supermarket parking lot on a 95-degree day with newly bought frozen food in the cab. I have since discovered that I can run nonstop round trips, for example, driving to a post-office drop box and back. Not very practical.

I should mention that the trouble occurred after the water pump failed and the engine overheated. The water pump has been replaced and no more problems with overheating/staying cold.

[Speaking of codes, whoever wrote this computer software isn't a car guy, because the spell-check doesn't recognize "driveability."]

 

[Bob Lincoln]

Well, both the coolant temp sensor and the temperature gauge sensor are right above the water pump, on either side of the thermostat. They are in a position to have gotten cooked. Moreover, when I examined the sensors on my 92 Dakota, both had fractured bodies, and trying to unplug one pulled its guts out. So I replaced both, and all was well.
It's conceivable that the CTS is not reading properly, or is intermittent and maybe not storing a code, and is enriching the mixture when the engine is warm. That could cause a hot start issue. Is gas mileage affected?
More basic first pass - are these plugged in completely, and the wires and contacts in good shape?

 

[valiant67]

When it won’t start do you have spark? I had a similar situation in an older car and the pickup in the distributor would heat soak. Always started cold. Always started if you restarted within a couple minutes. If it sat more than a couple minutes, it would not restart until cold. No spark.

 

[Bob Lincoln]

In this engine, the pickup does not control spark. It controls the timing of the fuel injector firing. The spark plugs are fired off the signal from the crankshaft position sensor.

 

[valiant67]

In this engine, the pickup does not control spark. It controls the timing of the fuel injector firing. The spark plugs are fired off the signal from the crankshaft position sensor.

Yes, and the crank sensor could be getting heat soaked.

I think the first thing is to see if there is spark when it won't restart hot.

 

[chuzz]

The 3.9 was a great engine. But with that many miles on it, I'd be looking at the distributor.

 

[dana44]

Agree with Chuzz. The distributor still has a Hall Effect in it, and their known problem is to die with heat soaking, cool down and work for a bit, and do it again. It is a little different that it will run and run, fine if not shut off, but hey, different things have happened. The coil itself, which can also do a heat problem dying thing, is located next to the water pump under the exhaust manifold and in a pretty hot area, too.

 

[DrDan68108]

Thankfully most of these components are reasonably within reach. However, where's the crankshaft position sensor?

Just gave it a test, and the no-start is also no-spark (tested @ plug).

 

[AllanC]

. . . . However, where's the crankshaft position sensor? . . . ..

Here is a Y T video that shows how to access crankshaft position sensor.

View: https://www.youtube.com/watch?v=lbUxwexlyf8

 

[ImperialCrown]

If it is losing spark when hot, it may be the CKP (crankshaft position) sensor. They may not be easy to get at. Being underneath with extensions and a flex socket may help.
Like Bob L. says, I have found coolant temperature sensors that have crumbled to bits when I pulled the connector off. If the sensor is nearly open, it may tell the PCM that the temperature is -40° or some such nonsense.
The PCM may command a rich cold-starting mixture that a hot engine may not be able to fire? Sometimes holding your throttle to the floor will admit enough air to fire the rich mixture.
Since that minus forty reading may still be 'believeable' to the PCM, it may not set a fault code or light the 'ck eng' light. If you were to remove the connector completely (fully open) or ground it (short), it should set a fault code and light the 'ckeng' icon.
There may be 2 temperature sensors adjacent to the thermostat. One is for the PCM and one is for the dash gauge. The one with the Vt wire is for the gauge. You want the other (PCM) one.

 

[dana44]

Thankfully most of these components are reasonably within reach. However, where's the crankshaft position sensor?

Just gave it a test, and the no-start is also no-spark (tested @ plug).

Back edge of the engine, front edge of the transmission, passenger side top. There is a notch in the transmission case. You can access it from the top of the engine probably a little easier than what the video itself shows.

 

[DrDan68108]

Update: the truck wouldn't start today and it's plenty chilly. I tested for power from the coil, securing it where it could jump to ground without zapping me - nada. Distributor has nothing to distribute.

I can see the PCM from above; it has greenish deposits where it meets the engine - copper oxide?

 

[Bob Lincoln]

Not copper oxide, but a copper salt - sulfate or chloride. Are you talking about the connector and its contacts? If so, that's easily enough to kill it.

 

[DrDan68108]

Not copper oxide, but a copper salt - sulfate or chloride. Are you talking about the connector and its contacts? If so, that's easily enough to kill it.

The stuff is on the hex head where the gizmo screws into the engine.

...
There may be 2 temperature sensors adjacent to the thermostat. One is for the PCM and one is for the dash gauge. The one with the Vt wire is for the gauge. You want the other (PCM) one.

I'm assuming this one is for the temp gauge on account of the wire color? The alternator appears at the bottom of the picture.

 

[ImperialCrown]

Yes, Vt/Yl would be for the instrument cluster. I don't have this specific service manual, but the ECT sensor for the PCM may be Tn/Bk and Bk/LB?

 

[DrDan68108]

Yes, Vt/Yl would be for the instrument cluster. I don't have this specific service manual, but the ECT sensor for the PCM may be Tn/Bk and Bk/LB?

Yes on the wire colors. Found the diagram online for my specific year.

I'm wondering if the real problem is the crank sensor because of the truck no longer starting when cold, either.

 

[Bob Lincoln]

A retaining screw showing corrosion won't be an issue. I presume you mean either the screw that holds the harness connector to the PCM, or to the fender. The computer doesn't attach to the engine. But I would still disconnect the battery, then the PCM connector, and inspect the pins and sockets on each side of the connection.

 

[ImperialCrown]

The crank sensor triggers the PCM, which turns the negative side of the coil on and off, which causes the coil to spark.
A problem at the temperature sensor would not cause a loss of spark.
As I remember the coil is mounted right next the the passenger, front of the exhaust manifold. The heat from the manifold caused a shortened life for many of these coils.
Also being mounted behind the radiator air stream, they got a good blast of wet weather.

 

[DrDan68108]

The crank sensor triggers the PCM, which turns the negative side of the coil on and off, which causes the coil to spark.
A problem at the temperature sensor would not cause a loss of spark.
As I remember the coil is mounted right next the the passenger, front of the exhaust manifold. The heat from the manifold caused a shortened life for many of these coils.
Also being mounted behind the radiator air stream, they got a good blast of wet weather.

I'm inclined to start with the coil. It also got a good steam bath when the engine overheated. (Besides, it's the easiest of the suspect parts to access.)

 

[Adventurer55]

I had exactly the same problem with my 96 Dakota. I replaced the crankshaft sensor first because I was getting some misses at road speed. That fixed that but the no start when hot turned out to be the pickup inside the distributor. It happened this summer.