Engine/Transmission (1994 - 1998) Low idle when hot and heavy load

[fulltimer]

My idle goes way low when I have my truck camper on and have

driven for awhile. It is normal when engine is cold.

Any ideas.

 

[Bug Out]

Mine does the same thing, but

My idle drops down about 100-200 RPM's too after a long hot run and temps are up outside,and or one or both.



My engine dies if you let off the throttle without tapping the throttle to keep the R's up. . A/C on don't help matters either!



It will die quick if I leave the exhuast brake on slowing down if not tapping the exhuast.



This make for loads of fun with a 15,000 # 36' Alpenlite behind me and the brakes stop working after the engine dies.



The pucker factor kicks in quick when that happens. . LOL It all started after the #11 plate was installed.



I'm learning to deal with it.

 

[Missouri Mule]

fulltimer and Bug Out,



What are your tachs reading at hot temps?

What are your tachs reading at regular temps?



It seems to me you need to raise your rpm at idle with ac on.

(especially you Bug Out)

I have read that hot weather would cause lower hores power because of air expansion. I have read the cool weather will increase horses a little. But should not have any effect on rpms at idle.



I would think a worse case scenario would be fuel pump starting to go ( lower pressure when engine is warmer).



Full Timer, what is trans temp readings before /after hard run?

But than again could your trans be on its way out?



Just thought of something else. Have you let the cat out?

 

[JIM S.]

I have noticed that same thing..... last time I pulled my 5er (2 months ago) when I got home the idle was way low. trans temps never got above 180 (incredeble since my TC slips some now days) and my normal idle speed is right on spec.



I was wondering if a dirty fuel filter might cause some of it, what do ya'll think?

Jim

 

[fulltimer]

The RPMs are normal when the engine is cold. And they are

down so low that the engine is running real rough after

a long hot run. I do not know the exact numbers. I would

say from the tach on the dash that it is about 700 at

normal and about 500 when down low. It is not the fuel

filter because I have changed it. When you say "let the

cat out" I guess you mean take the catalytic converter off.

No, it is still on. If that was clogged would it only show up

when hot? or at all times? The engine does not seem to

be losing any power. The trans temps are normal. And

the exhaust gas temps do not go above 800 dgrs.

 

[radixr]

Hot Idle

Apparently it is normal for the idle to drop some when the engine is hot (mine does it also). Why this would happen, I don't have a clue. If you have idle speed set correctly, the engine should not stall under any normal conditions. According to the shop manual, idle speed should be set to 750-800 RPM (automatic trans. in drive/AC on) or 780 RPM (manual trans in neutral/AC on) and engine at normal operating temperature.



I have mine set at 875 RPM so it doesn't go below 780 even after hard running.

 

[Missouri Mule]

Full Timer,

Your idle when engine cranks seems to be to low. I going to look idle rpms up in service manual.



OK from 96 Dodge service manual;



Low idle speed. (High idle speed is for max fuel flow and really has nothing to do with how engine idles).



Automatic trans = 750 to 800 with trans in drive and ac on, also at normal operating temp. You mite want to adjust a little higher.



Manual trans= 780 rpm with trans in neutral and ac on, at normal operating temp.



Mine is set at about 825 to 850 with out ac on soon after start up. ( 5 spd trans) It was set at this rpm when I got the truck.

 

[illflem]

Are you guy's fuel levels below half a tank when this happens? I had much the same problem and my theory is that as the fuel temp increases the spring in the overflow valve loses compression strength to the point that fuel pressure drops. I have a fuel pressure gauge and have seen the pressure drop below 10 psi when the fuel's hot. I've seen temps on the outside of a partially filled tank as high as 190° after a long pull!!! I even had a passenger keep the idle up when it was idling low while I added fuel to the tank, couldn't have added more than 3 gallons before the idle and fuel pressure returned to normal. It has to be fuel temp related. Changing the overflow valve hasn't helped me but keeping the tank over half full for greater dilution of the heated return fuel has. In fact there have been no problems whatsoever with a fuller tank. The addition of a fuel cooler on the return line may also work but keeping the tank full is cheaper/easier, stations with diesel aren't that far apart. My half full by the gauge isn't half full gallon wise on my truck, it takes about 22 gallons to fill from half. Piers also suggested the problem could stem from a weak fuel shut down solenoid partially closing when hot, but this leads to problems at higher rpms also.

 

[dseabaugh]

I p;osted about the idle dropping a couple months ago. Finall got to the point of diying when the throttle was released quickly. I put in new throttle valve ($50 bucks wasted) and set up idle speed. Now it doesn't die but is a pain to drive with idle set up as the truck is ready to go. On flat ground it will get up to 30 + mph with no foot on the throttle. Pump has maybe 30K miles on it. Idle still drops 100 rpms or more after a pull vs. cold. The only good thing I accomplished is it doesn't die and when I took apart the old unloader valve I found it to be assembled incorrectly. makes me wonder if injecter pump is oil cooled and oil is at higher temps after a pull causing expansion in pump affecting clearances or if fuel is heating up and thinning to consistancy of #1 instead of #2?

 

[Missouri Mule]

While talking about hot fuel how about if the fuel heater is in operation when it should be off, mainly because of bad sensor or relay?

 

[illflem]

Good point mule. Of the heaters I've seen fail in that manner all have fried the wiring, guess they're not designed for continuous duty. Once the wiring fries the heater is permanently off. Not sure what temps the fuel heater is supposed to run at but know that it doesn't need to run long. I've seen the bottom of my tank go from 20° to 110° in 40 miles on a 20° day. The injection pump seems to be a very effective heater once things warm up.

 

[Missouri Mule]

illflem,

As I recall fuel heater is turned on at 40 degrees and is turned off at 80 degrees. It is I believe to keep fuel from gelling on cold nights/days when engine is off. But if that sensor starts to go bad!

As you stated there have been quite a few heater wires fried. But that mite be caused by the heater element starting to go(pulling to much juice for the wires to handle).



I would think that air movement under truck would cool off(pull heat from) the fuel lines and tank.

But injector pump or the fact that the return fuel line runs along side of head mite be cause for very warm fuel.