New to 1st gen tech q's? idle adjustment

Just picked up my first gen truck today...drove it 350 miles home and found a few issues with it. Its a 1991 two wheel drive auto trans dually.
1. It seems to idle WAY to low? with no tach I can't tell but it sounds to be around 500rpm's Barely enough to keep it going at a stop light. Where on the pump is the idle set screw?

2.The brake pedle is spongie & works it way to the floor coming to a stop light if you don't pump it.

3. Low oil pressure light has come on several times while sitting at a stop light. I can't tell if the truck is really loosing oil pressure or if its a bad guage. The truck shakes and rattles a good bit I kind of think its from the low RPM or bad dampener. Where can I plug in a oil pressure guage to test this? and what kind of pressure should I see at idle? The light goes off if I put the truck in neutral or park at the light and gently rev the truck up....along with most of the vibration it basically smoothes out.

4.Water pours out from the venting that is underneath the glove compartment when I have the A/c on and I come to a stop from highway driving. Im sure its just a drain hose that is missing or cracked? could it be something else?

5.When the truck is in park it reads that it is past park on the column pointer thing. When its in drive it says its in Reverse. Can this be adjusted without to much trouble?

6.Truck seemed to top out WOT at 70-72MPH is that from being a dually? or possible just govered at this speed?

I appologize for the long post this is my first ever 1st gen cummins. Im used to P-pumps & CP3's

Lee Majors

9 LIVES said:

Just picked up my first gen truck today...drove it 350 miles home and found a few issues with it. Its a 1991 two wheel drive auto trans dually.
1. It seems to idle WAY to low? with no tach I can't tell but it sounds to be around 500rpm's Barely enough to keep it going at a stop light. Where on the pump is the idle set screw?

2.The brake pedle is spongie & works it way to the floor coming to a stop light if you don't pump it.

3. Low oil pressure light has come on several times while sitting at a stop light. I can't tell if the truck is really loosing oil pressure or if its a bad guage. The truck shakes and rattles a good bit I kind of think its from the low RPM or bad dampener. Where can I plug in a oil pressure guage to test this? and what kind of pressure should I see at idle? The light goes off if I put the truck in neutral or park at the light and gently rev the truck up....along with most of the vibration it basically smoothes out.

4.Water pours out from the venting that is underneath the glove compartment when I have the A/c on and I come to a stop from highway driving. Im sure its just a drain hose that is missing or cracked? could it be something else?

5.When the truck is in park it reads that it is past park on the column pointer thing. When its in drive it says its in Reverse. Can this be adjusted without to much trouble?

6.Truck seemed to top out WOT at 70-72MPH is that from being a dually? or possible just govered at this speed?

I appologize for the long post this is my first ever 1st gen cummins. Im used to P-pumps & CP3's

Lee Majors

Click to expand...

1. There are two ways to turn up the idle. The way you're supposed to do it is by screwing in the "low idle screw", which is a screw that the "throttle" assembly rests on. The screw is between the head and the injection pump and a !@#$%$##!% to turn. I use the box end of a 1/4" wrench to turn it, you're mileage may vary. The wrong (yet fun) way to turn up the idle is by screwing in the full power screw, which will give you power, smoke and heat at the same time as raising the idle.

2. Brakes on these trucks suck. I've replaced everything more times than I care to admit. Let's just say that if I spent $3000 on a totally aftermarket brake system when I first bought my truck two years ago, it would've paid for itself by now.

3. I've heard a few reports of the oil sender being a bit too sensitive. In general, oiling problems are rare on these trucks. I would try to bump up the idle and see if the lights go away; it's probably nothing.

4. I took my air conditioning out so I could go faster. :nos:

5. The gear indicator can be adjusted pretty easy. A string ties the slide to a bent piece of metal on the column. All you do to adjust the slider position is to bend the piece of metal. I think that you have to take the dash off to get to it; that requires the removal of the six dash screws, the placing of the wheel in the lowest position, and the placing of the transmission in 1st. Then you gotta jiggle the dash bezel until it unclicks, and then you gotta be Mr. Flexible to disconnect the "Message Center".

6. You're likely not getting full throttle travel. Otherwise, 3200 RPM springs do pretty good.

Now for some notes:

Is this truck intercooled and overdrive? 91.5 was the year they made a lot of changes.

A drop in RPM's while warm is a good sign of a worn out injection pump.

X2^^ Put Very Well Begle. The oil sending unit is threaded into the block just behind the injection pump. I just recommend the unused oil port on the top of the filter houing to pop in a test gauge. It's 1/8" NPT.

must be a 91.5 its intercooled and has OD.


Thanks for the help...im going to go turn up the idle first and see if that helps.

or turn the full fuel screw until the idle is right.

What is the best way to get the fuel screw out of the back to pry that welded on collar? I rather not take the pump apart to get a good reach at it.

I always just unscrew it. cut the lock collar off and reinstall, try to keep the nut about the same so it does not rev to the moon when you restart. Then just set fuel screw where you want it.

Some people have a lot better luck with the collar than others.

I beat the crap out of mine with a chisel, heated it up and beat the crap out of it some more, then I had to buy the funny metric tap to clean the threads. And then it broke a week later, probably due to my heating it and beating the crap out of it.

Long story short, it would have been worth it in the beginning if I would've just bought a new Bosch screw from the Cummins dealer that didn't have the collar.

$20 for one with no collar and a hex head.


I've also ran mine away the three times I put the fuel screw back in. Definitely use a 2x4 ready over the intake after you put the screw back in.

What would make it want to run away? I turned it roughly 1 full turn with no difference? also turned the star wheel with no difference. This thing is a major pig with a top speed of 70 mph. I guess that's a dually for you

How much fuel can I turn up on this truck before my tranny is in jeopordy?
I just want to be able to drive along with traffic instead of holding it up.

As you turn in the full power screw, you're going to increase idle and it's going to start accelerating by itself after a certain RPM. With it out, you can take it to 3000 RPM in neutral and hold it there; as you turn it in, the maximum RPM that you can hold it begins to decrease. One turn and it's going to start increasing RPM by itself above say 2800 RPM; two turns and it's going to fuel itself whenever you go above 2600 RPM, three is 2400 RPM, and so on. In neutral, mine takes off by itself if I take it over 1100 RPM.

It stops fueling itself and comes back down when you let off on the pedal; when you're really close to runaway, it is going to come back down really slowly.

Runaway is where it fuels itself at idle, and it won't come back down no matter what you do with the pedal.

As you go, you turn down the idle with the low-idle screw in between the pump and the head.

To stop runaway, have the air hose off the intake and something solid and smooth ready to put over the intake.


I don't think the stock transmissions are that bad unless you're towing with them or have an aftermarket converter. The stock converter is loose enough that I think it preserves the transmission. I had no problems with my stock transmission and a full power screw, until I wore through a band, and that probably would have happened anyways.

You do need a pyrometer with any tweaks to the fuel screw.

For driveability, make sure that your overdrive is working and that you have full throttle travel. Then get a 3200 RPM governor spring.

the AC... under the hood there is a little condensation drain hose coming from your HVAC box... it will get clogged with crud and then when you turn or stop all the water in the box sloshes onto the floor.

turn that power screw, turn the fuel pin, loosen up the AFC, and put a governor spring in it!!! 70mph top speed will be a thing of the past

I don't even think the truck has full RPM travel with the throttle pedal. I held it in neutral and from what my tuned ear can feel it sounds to be somewhere around 2,100-2,200RPM At WOT at the pedal.

what is the redline on these old bastages?

about 2200

JK, they're pretty gutless stock...

Im just now learning that:bang

I would be somewhat thrilled if I could atleast get this rusted heap to keep up with traffic....maybe somewhat par with a stock 2 gen 12V. :hehe:

when we bought the 91 new my wife found out abt the 2nd time she drove it abt the top spd. she pulled out on a 2 ln to pass and nearly got killed. she drove it around and found it would only do 68. she went to the dealer and started hollerin'--real loud. they turned it up to abt 75 but they said it would explode or something any faster than that. turns out the dodge dealers didn't know anything abt the cummins. we started taking it to a cummins truck shop for work.

Yup, the 3.08's with the 727 were good for about 72 to 75 stock. I can do 65 in second now.

My truck would do about 74mph foot to the floor, with about four turns on the full power screw I could do 85mph. The oil light is cause it is idling so low. Crank up the full power screw more and that should solve most of your problems. Each turn on mine was worth about 15hp.

Since your truck is a dually it has 215/85R16 tires, right? With those short tires and with overdrive (0.73 IIRC) and 4.10's, top speed should be about 75 with the pump set for the factory governed speed of 2600rpm. I would check your throttle linkage for proper adjustment. Make sure that the threaded rod on the fenderwell side of the IP is adjusted so that any movement of the fun pedal leads to movement of the arm on the pump. Also, make sure that you are getting full travel of the throttle cable by checking to make sure that the pump lever is being pulled to the high idle screw by the pedal. If it isn't, it will lead to low power and reduced RPM due to the pump not being pulled to full fuel. Also, throw out the high idle screw, it really isn't necessary.

As a side note, when I bought my 92, it was a laggy, slow beast that made only 15psi of boost. Since yours is apparently a 91.5, it has the 21cm turbine housing (same as my 92). Get a pyrometer on there, install a better torque converter, and crank the fuel screw in. A smaller turbine housing (the 16cm^2 housing is a popular upgrade) will help spool up and make the truck feel faster.

http://tinytach.com/

This is what I used for a tach on my first gen. Cheap and it works.