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Discussion Starter · #1 ·
Hello. I have a 91 D250 with auto trans, VE injection pump, 98k original miles. I have a very low idle when the engine is hot. The problem usually occurs when towing or after a long freeway drive during hot weather. When I stop at a light (still in gear), the idle drops so low that I lose all oil pressure. The idle improves if I put it in neutral or park. It has never stalled, but it barely runs and is a bit harder to restart if still hot. However, it's fine after the engine cools for a while. I have no other symptoms and the truck runs great overall. It's only an idle problem. The problem rarely occurs during the winter.

What's could be happening here? It has a newer lift pump and fuel filter. The valves and idle speed were recently adjusted by a Cummins shop. I also checked the electrical connections on the solenoid, temp sensor, etc. I don't know? Any help would be appreciated. Thanks.
 

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The bad news is that's a classic sign of an injection pump on its last legs.

Do you have a tachometer? Do you know how low the RPM is dropping? A hundred-or-so RPM drop is normal... Depending on where the idle screw is, I'd consider bumping it up a bit just to live with it.

The good news is that the oil pressure sender has a tendency to be a bit hyperactive. You're probably not doing any damage on that end.

Maybe out-of-spec injectors could do it?

I'd getting prepared for a $500-700 rebuild in a few thousand miles, unfortunately.
 

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Discussion Starter · #3 ·
No, I don't have a tach, but will soon. I've been preparing myself for a pump replacement, but I'm still hoping that it's something else. I plan to troubleshoot the sensors and torque converter this weekend. I'm open for any suggestions or ideas. Thanks for the reply.
 

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Discussion Starter · #4 ·
New injection pump installed

I broke my wallet and installed a new BD Power injection pump and gauges last week. What a difference!!! Much more power and it idles waaaaaay better!!! Cold starting is also better now.
 
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