Engine/Transmission (1994 - 1998) Rough running engine with A/C on in high ambient temps.

[Gator]

Morning folks. I've got a 97 3/4 ton, AT, with 550,000+ miles on it. The problem I'm having is this: When the ambient temp is above 90*, and I engage the AC, it works well. Until I
come to a stop. The engine is idling like it's hitting on 5 cylinders. The sound coming from it is also like a water pump cavetating, trying to pump air. (retired fireman) When I shut off the
AC, the cavatation stops, but the engine still runs very ruff! This syndrome never appears when the engine is cold, and the ambient temp is cool. Obviously I'm not running the AC
either. The engine temp itself runs around 180*-190*. The fan engages and keeps it cool. I just replaced the radiator and condenser a month ago because they were old and beat.
One event that happened for the first time when I pulled into a fuel stop, was the fuel gauge needle dropped to empty and then came back to it's normal read.
This has happened 3 times now when I come to a stop with transmission in gear and AC compressor running. Take it out of gear with AC off, and the noise goes away, but idle is really ragged!

i took the trk to a diesel shop in Ca., and they spent half a day diagnosing it. They knew I didn't have the time to let them tear into it, just give me their thots. They came up with some
possibilities: tired P-pump, bad injector, or bad sending unit and pick up tube in tank. i replaced all fuel lines also before the trip.

One last thing. The truck has had a very rhythmic drone coming from the engine or? Gotten louder over the last year. Unrelated? Thx folks for any help you can offer. Gator
PS: I ran the trk today with full AC on in 86* temp. Nothing! Ran and idled fine................

 

[GAmes]

What is your idle RPM, AC on, in gear? As for "tired p-pump, I doubt it. Mine has over 1.2 million on it and is working great. If you have never replaced the injectors, then they are probably due. Piers diesel sells 215 injectors at a fair price. There are trouble shooting procedures for the fuel quantity gauge in the factory service manual. If you don't have one, Genos carries them.

 

[BigPapa]

What is your idle RPM, AC on, in gear? As for "tired p-pump, I doubt it.

I agree with this. Sounds like your idle is too low. Also, my pump has over 500K on it. It will lope with the A/C on, in gear, if the idle is too low.

 

[Gator]

Thx guys. I thot the idle was to low also, however, when the dsl guys checked it they said it was on target: 800 at idle, in neutral, and 600 in gear with AC both times. I drove it today in max 87* temp with full cold and full fan, and it didn't run as good as when there's no AC on, but it wasn't running nearly as bad as it did when the temp was 98*+.

BUT, what my rpm gauge is reading is 600 in gear w/AC on and 700-800 in neutral! Do I believe my needle or their instruments? Puzzled! Also, what about the fact that my gas gauge has, on three occasions, fluctuated from empty to full, then settled down. It happened when I pulled into the fuel station and idled in gear to let the turbo cool down.

 

[BigPapa]

I would suggest bumping the idle just a bit. Just get it to where it sounds right and doesn't lope with the A/C on in gear. You'll know when you get it right.

The fuel gauge is a separate issue and relatively common. There are several fixes on here from fixing and modifying your existing sending unit to replacing it.

I fixed mine in my '95 by removing the cluster lens, wrapping an old credit card I had trimmed to fit in black gaffers tape, and laying it over the fuel gauge. Then I pulled the "Low Fuel" bulb out and used the odometer to know when to fill up. Never had another problem out of it.

 

[GAmes]

The diesel guy apparently doesn't have a manual.. Idle rpm is checked with the engine at operating temp, in gear (if an automatic) with the A/C on. Spec is 750-800 but I keep mine at about 850 to keep the A/C cool in the summer while driving in stop & go traffic.

The most used tool in the shop. I prefer the paper book edition.

https://www.genosgarage.com/product/dfsm97-cd/dodgeram-factory-service-manuals

https://www.genosgarage.com/product/dfsm97-print/dodgeram-factory-service-manuals

 

[Gator]

Thx ! I'll make the adjustment

 

[SSage]

I try to idle at 850 to 900 rpm with my automatic. Strong torque converter and A/C on pulls the idle down 250 rpm or more. Steering pump loads it more too in tight spots, I lug down to 600 all the time backing trailers in even with a 900 idle. More than 900 rpm and it wants to take off too much, so I try for 850 to 900 rpm.