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idles way up,then back down....then WAY back up,then back down...etc etc HELP PLEASE

B

Badazz chevy

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Northeast, VT 05858 USA
:mad2: I just traded my 95 chevy Z71 ext cab 6.5l diesel for a 98 Dually ext cab 6.5l dually that i knew had an issue with the motor. motor only has 165,000. Not sure what has/was done before the problem. heres the problem.
P.S. I'm new to Diesel's. I build gas motors all day long thou...

I have to spray either into the air cleaner to get to even start. When it starts.the idle will go way up, then back down where the idle should be, then go way back up, then back down...... etc etc. I even tried givin it a lil throttle, but nothing. I then put it in gear... it will move in low range but jerks with the idle. If it was a gas motor, i would know where to start... but diesel is diff and wonderin where to begin my attack?
BTW...there are no engine light, but I DO have a snap on 2500 scanner !!
Please help. I traded my only running diesel for this and need a running truck .

Thanks Jeff :thumbsup:
 
First fix the glow pluigs and glow plug relay so that they work as they should.
You should under no circumstances be using either on an engine with working glow plugs.

Second check the lift pump function by opening the water in fuel tee-valve with the engine running and check for fuel low and note changes to how the engine runs.

Clean all grounds and verify good ground connections.

Lastly have on hand a working PMD/resistor/heatsink combo to try to solve the drivability issue.
 
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First off never use ether.

On the coolant cross over there is a T valve with a hose attached, open the valve with it running or turn the ign to on and a good stream of fuel should come out. The fuel will flow only as long as the WTS light is on in the second scenario.

Do your glow plugs work?
 
Welcome here.

I remember reading that the armature or something like that was at fault when a 6.5 revs up and down. Not sure, but if it's that you need to replace your IP. Just my 5c worth. I'm sure someone smarter will join in and add more info about this.
 
do all the other stuff first before jumping to the IP,very well could be the IP just want to elimnate the other stuff first
 
Do you only squirt it when its cold? after the engine is hot it will start without either? If yes, you need GP system checked out.
The idle issue could be a PMD. What kind is on there now? remote mounted? Or a long shot could be an obstruction before the turbo.
 
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Plenty of possibilities. can you define way up idle, it is really high like 3000rpm? Or high as in 1200rpm?

This can happen if the IP was installed way too retarded, or severe "optic bump" can make a bad situation worse. Need some scanner data, do you have access to one?

Or the coolant sensor on the t-stat crossover can cause idle rpm issues, you can try unplugging it and see what happens.
 
HELP !!!! =)

Sorry took so long to get back. i checked my email and nothing. not sure why ?
as for the truck. the guy drove it to my house before we swapped trucks. it ran fine then. long story short....i still traded figuring i work on car alot, and ...with a lil help could figure out the problem. One problem....i don't know much about diesel, but really want to learn.
As for the starting... not sure about the starting when warm, i can tell you it will get up to temp and has no affect on how the motor runs.
as for the idling...it goes up around 3k then back to about idle, then back up...then back down. Almost like somebody was running and breathing hard. Not sure if that helps.
even in gear it lurches with the idle. useing the throttle doesn't help... No effect.

Thanks Jeff
 
You really think so? From what i heard about PMD's is... they will start and die, then it might start and run for awhile and die again.
Will PMD's do this too. it has never died on me yet. but just "Breathes" hard. I don't have alot of money to burn on chasin parts... but if you really think thats the problem...i will go get one ! Know any good places to get one...Cheaply of course. LOL

Thanks alot !!
 
Well, to surge to 3000rpm, theres only a couple ways, since the IP is electronic.

The PCM is rather robust, uncommon failure point, and there is no idle setting that high. A false APP (accelerator pedal) input into the PCM could be an issue, but also not common, a scanner would show this, and would set a code if one of the 3 sensors in the APP had gone rogue.

That leaves the common PMD failure, which can cause surging, this can happen if too much current is put across the base of the transistor. Such as, if a resistance was failing and shorting to the emitter, or the transistors internal resistance had broken down. Ignition voltage swings could have a small effect, or the voltage regulation within the IP to the transitor base. Like if someone zapped the car with too high of voltage and it fried the PMD's voltage regulation capability to the transistor base. This has happened to a number of people.

The IP itself could be causing it, a fuel solenoid windings could have lost insulation resistance, or the internal transfer pump is not regualting somehow. The IP's drive shaft could break and cause this sencario, but be constant.
 
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thanks..... and i do happen to have just picked up a nice MTG-2500 Snap On scanner with a diesel troubleshooter pack on it. Its new to me, might take a few mintues to figure out...lol
Now question for ya...... i want to work on this tomorrow. If this was your truck where would you start and how/what would you do.
 
Anyway of testing a PMD or is it just change out and see if it works ?

Ohh ya...you said something about a voltage spike...like maybe the guy i got it from jumped it and maybe shorted it out or something like that ?

as for the PMD...i'm thinkin its all bone stock and mounted on the IP.............
 
Be warned that the Snap On scanner may not display everything correctly. I cannot speak to its capabilities. However I would look at the fuel rate and desired idle values. If the PCM is commanding a steady 10mm3 or less it should never go to 3000rpm, which you can also see by the desired idle value.

If it is constantly going up and down, probably have a broken IP. The PMD can be tested by using this test, which is mostly paraphrased from Accurate Diesel's site, but also available in GM troubleshooting guides in the technical library, and my own information added.

Those wishing to see if the PMD is outputting control signals to the injection pump, perform the following:
You will need to access the red wire in position "B" of the PMD connector with the PMD plugged in. This can be challenging with the intake manifold installed if using an IP mounted PMD. Straighten out a small paper clip and push one of the pieces of paper clip into the BACK of the PMD connector right alongside the wire to pin B. Push the piece of paper clip gently between the rubber seal and the wire. Wiggle the paper clip and push gently until about an inch of the paper clip is no longer visible. The paper clip is now touching the back of the bare metal terminal connected to the wire. Use your ohmmeter on the continuity test setting to ensure that the paper clip is indeed making contact with the respective wire. If you have wire piercing probes they can also be used for a more reliable connection (be sure to repair the wires later). You can also strip sections of the wire and attach short sections of wire as "test leads."
Now, plug the PMD connection back in. Connect your volt meter to the pin B wire and the ground on top of the IP, with the meter to measure AC volts. Be sure not to let the paper clip short against engine ground or the PMD will be destroyed!
Crank/Run the engine and observe the voltage. You should observe at least 1.0 VAC while cranking, up to 1.3VAC. If the engine will not start and this value is lower then the PMD is likely bad. If the engine will not start and your voltage value is higher than 1.3VAC then the connections to the IP's Fuel Solenoid (FS) under the rubber boot probably have broken loose. You can ensure the connections are good by measuring the resistance from PMD pin B to pin F (or FS ground), which should read approximately 0.4ohms on a healthy FS.
While idling you should observe approximately 1.6VAC. The voltage will rise with RPMs, up to 5VAC around 4000rpm. If your PMD is outputting a constant 1.6VAC, but the engine is revving uncontrollably then the IP is faulty.

This does NOT mean the PMD may not later cause stalling or other issues, but it should bring the engine to life and operate.
 
I have MT2500 and other than the TDCO setting I've not found any errant readings with it compared to GM T2, Carcode, or Matco scanners so don't worry about that it's a reasonably accurate scan tool for everything except TDCO and once you learn the quircks of the display it works for that as well; as for the troubleshooter cartridge I can't advise you on that as I don't have one for Diesels.

Most common cause of runaway engine or errant rpm more so than PMD is broken or worn armature shaft in the fuel solenoid in the distribution head of the IP, though it could be PMD since that is what drives the fuel solenoid.

To test with fully warmed up engine >170F coolant slowly advance throttle to 2000 rpm hold for about 5 sec or so and slowly reduce pedal back to idle, rpm should reduce as pedal reduces if it fails to return to idle or is jerky then your IP is need of replacement, takes a little practice to get the feel of how to run the test since you are "drive by wire" with the APP run the test 5 or 6 times to check it.
 
I have a TMG-2500 Was told it was the better model. Not to sound dumb...but what is TDCO mean ? And i can't hold the truck at a steady anything...lol throttle doesn't affect the rpm . when i try to gice it throttle...it just "breaths" harder but still goes up and down. I've been reading up on these motors and not heard of any doin this ? Any way of checking the solenoid ? sorry to ask sooo many questions.

And Buddy thanks for the info...i will be messin with the truck later today and will try about ANYTHING .. :)
I just know how $$ parts are. The IP would hurt the most !! lol
 
Badazz chevy said:
I have a TMG-2500 Was told it was the better model. Not to sound dumb...but what is TDCO mean ? And i can't hold the truck at a steady anything...lol throttle doesn't affect the rpm . when i try to gice it throttle...it just "breaths" harder but still goes up and down. I've been reading up on these motors and not heard of any doin this ? Any way of checking the solenoid ? sorry to ask sooo many questions.

And Buddy thanks for the info...i will be messin with the truck later today and will try about ANYTHING .. :)
I just know how $$ parts are. The IP would hurt the most !! lol
Click to expand...

The test I wrote out is to identify whether the issue is the PMD or IP's Fuel Solenoid. No other way to test it, other than replacing PMD and seeing if that fixes it.

TDCO is Top Dead Center Offset, its how the DS4 IP is timed. You'll have to spend a good amount of time in the technical reference area.
 
You sound like you do this alot. I will work on some of these today and let you know what i find... Thanks for all the help you, and EVERYBODY else.
Not stop with the info thou...lol I'm a quick learner.

Jeff
 
If you can't hold rpm and do a 2000rpm test according to what Bill Heath who taught me the test over phone a number of years ago, and the ones I've tested this method IP are in need of replacement. Diesle Pro from Pensacola Fuel Injection also uses this test.

This type of fail to fuel solenoid can be instant if armature shaft breaks, or gradual, from my experience the ones that are gradually worsening start doing errant rpm bumps/runaway when at idle & in park shifting to reverse, or at stop light holding brake on and you get a rpm surge.

BC run test Buddy points out see what that shows since you can't hold rpm to do the 2000 rpm test, but from description I'm thinking it's IP time for you
 
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