Jim Lane

Intermittent low idle after sustained high speeds, not always but sometimes when I get off the freeway and I had just been cruising 75-80 MPH for 15-20 miles I will get the the end of the offramp and be idling at a stop and the engine will feel like is going to stumble, it has a low or rough idle, if I put it in neutral and rev it a bit it will clear up and the idle will be back where it belongs.

Not sure how many RPM's I am turning at 80-MPH with 3.07 and a 727 but I am sure I have something wound up tight.

So what could be causing the low idle, torque converter?

Or maybe I was thinking possibly high head pressure in my A/C since my AC is putting out 10* frigid air and it hasn't had a chance to stabilize.



Ideas?

Jim

Onemoparnut

If I have mine wound up and I let off the throttle quickly I will get a rough low idle too. If I let off the throttle slowly it does not seem to do it. I don't know what is causing it but sounds similar. Someone told me something about the cylinder pressure going higher that the fuel pressure in the injector for a split second and that will force air into the injector and cause the rough low idle. I don't know if that is true but it is what I have been told.

mtdoragary

lift pump?

Jim Lane

New lift pump but I guess it could always be bad.
Haven't checked the pressure yet.

wannadiesel

Low idle when hot is a classic sign of a worn head and rotor on the injection pump. It'll be OK for quite a while longer. When it stalls, then it's time to start pricing pump parts.

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93flatbed

I had close to the same issue. My fuel pressure was good, new filters, good fuel and a rebuilt pump. I ended up finding my pickup screen was all but plugged in tank. Weird thing was it only showed a problem with driving high speeds for longer periods of time. I'm not sure why I never really felt anything besides then because it should have been lacking flow all the time, but never the less cleaned the screen and all was good.

Not saying its your problem but it was mine.

92smokin blacky

I have the same problem sometimes and have always received this answer^

I'm guessing that could be it with mine cuz i got plenty of fuel supply. And it didn't change when I changed transmissions although the new tranny lugs the motor down a bit more when i put it in gear than the old trans did. Sometimes it will bring it down to 450-500 rpm and feels like its gonna die sometimes. I just kick it in neutral when it gets bad.

Rick 12v CTD

I seem to have the opposite problem. My idle is a little low when it's cold.

Jim Lane

What makes it only temporary?
After I pull away from the stop, by the next red light the throttle it picks back up and idles ok.

Ill have to try it with the air conditioning off.

What wears in the head and rotor? the plungers or the distributor?
I am sure my truck has had its share of dry fuel run through it.

Jim

Onemoparnut

I thought when the pump was worn enough to cause a low idle that the idle would stay low. Jim is saying that the low idle is intermittent.

Exekiel69

if it is right after You where driving at highway speeds could it be the fan clutch locking up for a few minutes then un-locks after the temps from the rad are lower.

SOOT4BRAINS

I notice a similar condition in my truck. When its hot outside the a/c and my fan seem to drag down the engine a bit. When it cools off a bit the idle comes up. The idle only drops 50-75 rpm. Not a whole lot but noticeable. A little more vibration too, no doubt from the compressor running. How do you get your a/c so cold?!?!?!?

cougar

My 91 would have a low rough idle after a long had day of towing and would clear up after a couple days of normal driving. I figured my injectors were getting fouled. So I started adding more injector cleaner in the fuel when I tow and that seems to have done the trick.

diesel402004

Both my 91 and 93 do the same thing; although, my 93 doesn't do it as bad. It always went away after driving street speeds for a few blocks (usually with the fan clutch kicked in). I attributed it to tranny temps being somewhat hotter at sustained speeds. Seems the common thread is auto trans?

ClassA4x4

My tranny temps are much lower at highway speeds due to the improved airflow. Then it creeps up after exiting into the surface streets.

Anyway, my gas Suzuki has the same illness but not too often. The idle gets low after sustained high rpm with the AC on. I think it has something to do with the AC compressor dragging the engine, because the idle goes back to normal if I switch off the AC. The idle returns to normal even with the AC on after a few minutes.