Idle speed

[holeshotracing]

What RPM should my engine be idling at after I start it? When my engine's idling, whether it's cold or warm, the whole truck vibrates, the windows rattle, etc. As soon as I take off with it, it quits, but it seems like anything under around 1000 RPM makes it shake like a wet dog. I took my boss, who's kind of refined and drives gassers, for a ride in it the other day, and he's like, what the heck's the deal? It was actually pretty cool, because with the vibrations and rattles and squeaks from the cowl cracks, and the slider being open to hear the exhaust, it kind of freaked him out.
I'm just wondering if I need to idle it up a little.

[jogl]

Mine idles with a nice smooth growl at 750 rpm. When first started it idles at about 700 but it is up to 750 after 1 or 2 minutes. Has yours always been that rough?

JP.

[JLeonard]

Yes my idle is at about 700 and it is very smooth. Perhaps you have an injector or two that are needing service??

[Greenleaf]

My 90 has the OEM injectors it left the factory with.

It idles like it has a gas engine with a performance cam. When I get out on the road it smooths out and @ 15/18 psi of boost we really start to go places.

[holeshotracing]

Has yours always been that rough?

JP.[/quote]

I've only had it a couple of months, 300 miles. (The NEXT 262K is going to take awhile!)

The injectors are fairly new, I think the p.o. said he put new OEM ones in at about 180K. He always ran MMO in it so would that keep them cleaned out? Maybe I need to dump in some Howes or something.

My husband started it on Saturday to get it in the garage to change oil in the Getrag (it was LOW when he checked it, God forbid!) and said it was sitting there idling, and once it just quit, like it was going to die, but then kept running. It hasn't done THAT before either.

I got rid of a '93 three months ago with 56,000 miles on it and then got this one, so I'm not used to all this stuff that's wore out on it!

Thanks everyone for your comments.

[Greenleaf]

My 90 has always idled very rough. I got it at 316K miles.

[JimmieD]

It's pretty easy to do some simple testing. With the engine running you can crack the injectors open [with a rag over them to prevent fuel squirting all over] one by one and see what it does.

Each injector when cracked open a little should make engine run rough, and smooth out when tightened. If there's no change then there's some problem with that particular injector.

Could be some garbage or debris got in there when injectors were changed that's now blocking fuel flow. Sometimes just loosening the fitting enough to let it pump fuel through will let it purge out whatever junk is in there, sometimes not.

If the engine is really in need of a proper valve adjustment this can cause it to run rough also. Mine runs a little rough and I can hear one rocker arm rattling so I know I need to get in there and adjust valves. You can probablly re-use the valve cover gaskets. A Cummins barring tool, available from several sources, makes it easier, but engine can be turned over with alternator nut. This turns it in opposite direction than barring tool, so you have to do valve adjustment in reverse order. I'm sure someone else can explain it better.

If none of this works you can do a compression test through injector holes to check the head gasket. There should be some overheating and other indications if this was the problem.

[dpuckett]

I've noticed the injector line crack method to be a poor method to check injectors. It will tell you if one or two is out of sync with the others, but what if they are all equally bad? I had a 93 one time that idled smooth, little smoke, but it sucked fuel (14-16mpg hwy). Injectors were clogged and popping at 2500 vs 3500 they should have been. Cleaned and reset the pop pressures, and it got 18city.

Timing will make it vibrate, as will a slight clutch misalignment. I'd get the injectors checked and go from there.

Daniel

[holeshotracing]

Daniel:
How do you tell if the clutch is out of alignment? My husband's going to be pulling my Getrag to fix the leaky rear main or whatever it turns out to be, so he could check. The P.O. put the clutch in at 216K--it's a Luk Pro Gold. Are they as bad as everyone says??

[dpuckett]

As long as it holds what you need, I'd leave it in there, unless you can find a Sachs for a good price (note I didnt say cheap).

You can check clutch alignment by getting an old input shaft (that is NOT worn out at the tip) and seeing how easily it slides in and out of the bore. The plastic alignment tools work, too, if you dont have 3 old input shafts lying around like some do.

The LuK ProGold I installed in a 92 W250 was a piece of canine excrement. Wouldnt hold what a stock Sachs clutch would hold with a GCVW of around 18K. The truck vibrated at idle after that install, too. Must have been something in the alignment or balance of the clutch assembly.

Have your husband pop the top off the tranny and check for wear and signs of slack in the bearings while he's at it. Or you can, doesnt matter. Better to do it while it's out.

DP

[JLeonard]

Another way to check for a bad cyl is to use a IR heat gun.
But if the injector was only off a little, like enough to make a slightly rough idle, I don't think you'd be able to tell by a heat gun or by loosening the injector lines.

[BEARKILLER]

I had a 93 one time that idled smooth, little smoke, but it sucked fuel (14-16mpg hwy). Injectors were clogged and popping at 2500 vs 3500 they should have been. Cleaned and reset the pop pressures, and it got 18city.


Daniel

I would assume to know the pop-off pressures, the injectors would need be on some type of pressure-testing device.

How did you go about cleaning the injectors??

Do it yourself; or, is this something best left to professionals?

Thanks.

[dpuckett]

I'd leave it to the pros if I were in your shoes. It requires a pop tester, along with a cleaning solvent or sonic bath cleaner, along with knowing what shims to install in the injector to set the pressure.

INjectors have to be out of the engine to test them.

DP