Engine idle rough - where should I start.

[draymorris]

I now have a 91, 1 ton, the odometer shows 207k but I figure it probably at least 400k on it.



I have driven the truck for about 3 years. It used to belong to my employer who is the the dirt work business. We used it to pull a tandem dual trailer that was primarily used to move the skid steer and backhoe.



The truck has "loped" at idle and probably on up to about 1500rpm since I have known the truck, but now it seem to be getting worse.



One person suggested an o-ring in the fuel pump. He also said that I will probably have to have the whole pump repaired.



Any suggestions?

 

[MMeier]

Anyone change the fuel filter in recent history ??

 

[draymorris]

I changed the filter yesterday and many times when I worked for the previous owner. It never has made a difference in the way the engine lopes.

 

[Tippin Spotted Cow]

Have you tried bleeding the injector lines? It's a long shot, but start with the simple fix first. These things have somewhat of a lopey idle when there's air in the lines, but it doesn't exactly even out when you goose it though...



I know a lot of these trucks that flat out shake like all hell at idle, but even out as soon as you hit the accelerator. I still don't know what causes it, because I've never seen anyone keep the truck long enough to fix it when it has an idle like that... . go figure.

 

[JLEONARD]

When I put new injectors in mine the idle became smooth as silk.

Old ones (100K on them) had carbon deposits built up past the tips and that changed the spray pattern and I believe that is what caused the slightly rough idle.

 

[PToombs]

Has the LP been changed? Maybe throw a guage on it and see what the pressure is. Low supply pressure will make them run funny too. I bet Jay is right though, 50/50 on supply or injectors.

 

[draymorris]

As for bleeding the lines - how long can air stay in the lines? This has been going on for years.



I notice that the lope is worse after a long hard run in hot weather.



Where is the best place to put a pressure guage on the lift pump?



How much should I look at spending on injectors?



Thanks to everyone.

 

[cerberusiam]

There is a fitting right in front of the top part of the fuel filter to tap for pressure.



The lope could be injectors, timing, or the pump itself. The injectors are the cheaper route that is th eusual problem so thats a good place to start. Find a Bosch certified shop somewhere near you and chat with them. They can rebuild your existing ones sometimes cheaper and better than you can buy elsewhere.

 

[draymorris]

Thanks for the help.

 

[PToombs]

dm, easiest thing to do is call Geno's Garage and get the tapped banjo fitting. (it's listed for 24v trucks but it fits fine) It's only $7 and saves lots of hassle. Just remove the banjo bolt with the bleeder and put the tapped one in there, then hook in a guage. You should have about 5-7 at idle and not less than 3ish at WOT.