Hey guys -

I'm a newbie here, but looking for some help with my truck (93 4x4 W250 LE model). It has a stalling problem at idle when the temperature under the hood gets high, either hot weather or towing. I can keep the truck running by applying some throttle, but once the RPMs get too low it just stalls no matter how much throttle I give it. The truck always starts right back up without a problem.

I searched this forum and found something similar related to a fuel shutoff solenoid having bad connectors, or a KSB (what's that?). You guys think higher temps under the hood would exacerbate problems with these parts? Any other ideas?

Thanks in advance for your help!
Chris

 

welcome first of all! a little more info on the truck would be helpful... are you running any gauges? when you say hot under the hood are you talking about EGT's? also is the truck an auto or stick? and if you have a tach, what RPM are you idiling at?

if your idle is low already and then you run the AC in hot weather it could cause it to drop too low. i haven't done anything with the shutoff solenoid but i'm sure others will chime in.

 

If your running AC with an auto you just need to adjust your idle up it should be a 750rpms with the AC on and in drive

 

Thanks for the warm welcome! I guess I should have added that extra info - sorry about that...

It's an automatic truck and doesn't have a tach (wish it did) - the idle seems to be a good speed, but I did try bumping it up some and that's not helping - makes the truck idle way too high during normal temps (i.e. starting up in the morning/driving to work).

By hot under the hood, I'm talking about the ambient temperature - if the air under there is hot from either the weather being warm (over 90 deg F) or from pulling a load (radiator exchanging lots of heat). I have an EGT gauge, and it's doing this regardless of EGT.

A/C doesn't really have an effect, although it will stall quicker due to the extra loading on the engine. It will still do it if the AC is off. Same thing with being in gear vs. neutral. It's almost like I can hear the engine RPM make a noticeable step down, like 150 RPM or so, and then it gets to the "point of no return" where no amount of right foot will recover it.

Supposedly this truck had a new injector pump put in ~6000 mi ago, so I'm thinking that's not the problem. I put in a new fuel lift pump (lever on the old one was broken) thinking it was just starving for fuel at low RPM when the fuel got thinner in the warm weather, but that didn't fix it either.

Thanks again.
Chris

 

How about a new fuel filter? Your problem is definately on the fuel side. Could be your loosing voltage at the shut down solenoid at lower idle so then it basically just shuts off. maybe after it dies take a volt meter and measure the voltage at the solenoid.

 

OK - that makes sense to me - if heat goes up, so does resistance, so a voltage drop at the fuel shutoff solenoid is entirely likely. I didn't realize it needed power to stay open - any idea what the voltage specs are? I also saw in another thread that the fuel shutoff solenoid is a pain to replace - true?

Fuel filter! Goes to show that it's easy to overlook the obvious - I've not changed it recently, so I'll give that a try as well.

Chris

 

I personally haven't had to replace one, if you have a crows foot it is probably easier, can't that bad. I would think that if you have at least 12V should be plenty. And also as the idle goes down your going to have less voltage along with the higher resistance.

When one of my fusible links went bad it didn't completely go bad. I would have power at the solenoid when I would first turn the key on but then a minute later I wouldn't have power. So for a while I thought I had power there cause you only check it right after you turn the key on so...

Good luck let us know what you find

 

I'm guessing NOT a fuel filter. These problems seems to come up at higher rpms and higher power. Doesn't take much fuel to idle.

Your idle speed will drop as air temps go up. I know you don't have a tach, but you want 750 rpms in D with the A/C on. It always sounds to fast to me when it's right. In the cold weather my idle goes up to around 900 unloaded. So...bump up your idle, and diffinately check your shutdown solenoid. Maybe just a loose wire that loses contact with the increased vibration of a low idle? These cummins will rattle your brains out if you let them idle too slow!

If it were a gasser I'd say you have a vacuum leak.

 

Definitely a vacuum leak! Don't fool with the other stuff.

10 volts is enough to keep the solenoid open. On the old wax motor KSB's I think they used the fuel shut-off solenoid to draw the voltage that powered the KSB down to 10 V . . .

I think you have a pretty good list to check here. If none of those are the problem try running extra lubricity additive in the fuel to see if that helps -- a worn out/sticking governor shaft can cause the same problem .

Good luck, and let us know what you find.

 

OK - I tested the shutoff solenoid with a hot wire straight to the battery, and that didn't fix it. It's not the problem.

So, I just started bumping up the idle until it didn't stall while in Drive with AC on. It sounds like it's ~750-800 RPM with that load on it. When it's in Neutral and AC off, it sounds like it's well above 1000 RPM at idle. I found a "friend of a friend" that has an optical tach, but it will take me a couple weeks to get it and check the truck out.

Yesterday was a good day to do these adjustments - it was above 100 degrees in the shade

Thanks for your help, guys!
Chris

 

Maybe you're one of the lucky ones that actually has a tight torque converter! Well, like I said before, the right idle always sounds to high to me. But it's what works.

 

I took the truck into a diesel shop finally - the injection pump is going bad. I will be replacing it later this week.
Thanks to all for your feedback here.

 

how did they determine that the pump is going bad?

Where are you getting a pump from? whats the cost going to be?

 

I suspect you're going to find the return springs (?) for the cam ring are going on you....when the pump gets hot the fuel thins and the springs loose even more of their strength...those things can cause a multitude of symptoms like yours....I hate to hear that on a such a "new" pump...bummer man...

BTW...if you want a really good "eyeball" method of setting your RPM it's easy....get in the truck, fire it up, crank on the AC, put it in "D" and look at the mirror on your door....yup, your mirror....if it wiggles a bit and the image is kinda fuzzy your under 750RPM...I bet you can get within 25-30 RPM using this one....it's worked for me when I've helped guys with no tach installed yet....

pb...

 

I had a return spring break on me once, pump would shut off sporatically. Crank it and if would sometime start right up or you had to crank it awhile. One rebuilt later and its as good as new. But broken springs and weak springs are two different things.