1990 Trooper 2.8 High Revs and Sparks Flying

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1990 Trooper 2.8 High Revs and Sparks Flying

Postby NC_Trooper » Tue Apr 27, 2021 5:04 pm

I recently bought 1900 Isuzu Trooper with the 2.8L V6 bone stock. Seemed to run fine. Had a small oil leak and needs a new exhaust. 200,000 on the odometer.
I got a little zealous and bought some parts just to make things fail proof, I guess. I replaced the TPS, TPS plug, IAC, plugs, and the driver side valve cover gasket due to a leak. I also replaced the vacuum lines. I did not use the OEM style rather I just got some from the parts store.

Went to drive the truck and idle problem started and I got a CEL. Codes 34 and 22.

The truck is idling high (2,500) once I start driving not when I first start up.

I took the new IAC, and TPS off and put the old ones back on because they were functioning fine and I initially replaced them for no real good reason. I found a vacuum line disconnected and fixed it. Cleared the codes by disconnecting the battery. Went for a quick test drive and 15 seconds in I miss a shift and go from 3rd to 2nd. Nothing drastic and I caught it immediately.

Then I hear a soft pop and sparks shoot out under the dash by my feet/legs. I get some smoke and a electrical fire burning smell. I pull the truck over and disconnect the battery.

I have found no evidence of burnt wires. There is nothing aftermarket wired into the car.

The idle is still high. What the hell is going on?!?
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Re: 1990 Trooper 2.8 High Revs and Sparks Flying

Postby DSUZU » Thu Apr 29, 2021 5:55 am

You can't simply unbolt and bolt on replacement TPS or Idle air valve. TPS has to be calibrated and Idle air has to be installed a certain way and it "learns" from there. Neither of these should have anything to do with sparks or smoke from under the dash. The ECM is in the center console, so even if you had hooked something wrong, the smoke (if any) would come from there.
You will want to hear from Ed.Mc or GeoffinBC as these two seem to be the best informed about 60 degree GM V-6s. Dennis
1991 LS Trooper 2.8 V6 5 speed conversion (Sold)
1989 LS Spacecab 2.6 5 speed 4x4 manual hubs, Rodeo torsion bars, extended shackles, 2" lift, K&N air filter (drop in), dump bed conversion, burgundy interior change, warn brush bar. Engine is bored .020 oversize and upgraded to 1994 pistons, head and EFI system.
Previously owned: 1987 Trooper II LS, 1989 Trooper (parts only), 1994 Trooper 3.2 Auto, 1997 Rodeo 2.6 5 speed, 1993 shortbed pickup 2.3 (project - sold) 1992 Rodeo 4x4 LS (parts car, devoured by Spacecab needs) plus some 70 plus other non Isuzu vehicles over the years. 1994 Amigo devoured to provide head and EFI system.
Heavily damaged by engine compartment fire while driving on March 24th 2018, condemned by insurance company and taken on March 30th 2018 :cry:.

https://forum.planetisuzoo.com/viewtopic.php?f=15&t=64060
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Re: 1990 Trooper 2.8 High Revs and Sparks Flying

Postby Ed Mc. » Fri Apr 30, 2021 2:32 am

Yeah, what Dennis said! IDK about your under-dash fireworks, the only electrical thing that's ever happened to me was in my '67 Mustang when the poorly-made-up connection to my EFI fuel pump burnt up while I was parking the car in my Uncle's driveway after a drive. The burnt wire hanging from underneath made it pretty easy to troubleshoot, and a hot wire from the (+) battery terminal to the fuel pump lead made the pump run good enough to get home.

You're probably gonna have to investigate under-dash with a strong light and maybe a mirror. Who knows, maybe it was a relay that popped. I can't imagine a wire shorted directly to ground, sparking and all that, and not a clue of it afterwards. An electrical component, perhaps.

Getting back to the engine, yes if you replace the TPS you have to do a recalibrate or you'll get all sorts of crankiness.

There is a specific procedure to drive-in the IAC so it's fully shut (not bypassing any air around the throttle plates), then unplugging its electrical connector, in order to be able to set the minimum idle speed. Once that speed (around 750 rpm for a manual trans, as I recall) is set, you have to check TPS voltage and adjust if out-of-spec.

Code 22 is TPS Voltage Low, so definitely needs adjustment.

Code 34 is MAP Sensor Voltage Low, could be because of the tuning issues, IDK. Be sure to check and see if the vaccum line to the MAP sensor is connected to the correct vaccum port, and isn't cracked and leaking. Any vacuum leaks could lead to a too-fast idle. Check all vacuum lines are hooked up IAW the vacuum diagram sticker on the underside of the hood.

Here's some good testing and troubleshooting info on GM MAP sensors:

https://troubleshootmyvehicle.com/gm/4. ... p-sensor-1

Below I've pasted-in an old writeup on adjusting minimum idle speed and TPS voltage.

The attached pic shows the plug that you have to pry out in order to get at the idle adjusting screw. The plug is soft, I used a small "tweaker" screwdriver to pry mine out.

HTH...................ed

Adjust yout TPS and I.A.C. on your TBI / MPFI

Authored by: Jay Vessels


Tools needed:

Torx bits or drivers (T-10, T-15, maybe more depending on the application)
Voltmeter (digital is best, but a really accurate analog will work)
Tachometer (the one in the vehicle will work fine if equipped)
Wrenches and an awl (various sizes, only if the idle speed hasn’t ever been set)


Theory of Operation – (lengthy)

A common myth about fuel injected vehicles is that the idle speed is fixed and cannot be adjusted. This isn’t quite
true; there is a setting. It's called minimum air, which is adjustable on TBI and MPFI vehicles. This setting sets the
lowest-possible idle speed for the vehicle. The ECM uses the IAC (idle air controller) to raise the idle speed from
this adjustment. So, while the exact idle speed isn’t really adjustable, the minimum idle speed is.

Why adjust the idle speed? Isn’t the ECM supposed to do that? Yes it does and it does do a good job, but has to
have a starting point. That starting point is called minimum air, or the smallest amount of air allowed to enter the
engine with the throttle closed. The ECM can only add air to that minimum setting. If that setting is too high, the
ECM can’t slow the engine down to an acceptable idle. If the setting is too low, the ECM may not be able to keep
the engine running under certain conditions.

Another reason to adjust minimum air is if there has been some repairs to the fuel system. If the throttle body has
been removed (i.e. rebuilt or cleaned) or the TPS (throttle position sensor) has been replaced or otherwise disturbed
(i.e. loosened the mounting screws unintentionally -- it happens) then minimum air should be adjusted. Any changes
that could affect idle speed or idle quality, like performance upgrades or replacing leaking vacuum lines, should be
followed by setting minimum air.

This adjustment, once learned, only takes a few minutes. It rarely has to be adjusted, but it takes so little time to
check (and adjust, if needed) that there’s no reason not to do so.



Checking & Adjustment Instructions

To establish minimum air, the idle speed must be set first. The idle speed screw is sealed with a cap from the
factory. This should be removed by removing the throttle body and using an awl to pry the plug off. If this seems
scary, have it done. It’s not difficult but it’s not worth risking damage to the throttle body or human flesh to remove
the plug. Once the plug has been removed, reinstall the throttle body.

Assuming the idle speed screw is accessible and the throttle body is installed, jumper
pins A&B on the ALDL (Assembly Line Data Link) connector under the dash. Pins
A&B are on the upper-right-hand side. These are the same two pins to jumper to
read codes from the ECM. Now turn the key on (the Check Engine light should be
lit) and leave the key on for at least 30 seconds. The computer will extend the IAC plunger all the way out to allow
adjustment of the idle speed.

(Note that on a Trooper, the ALDL plug is in the center console, under the pullout tray.)

After the 30 second wait, unplug the IAC (square 4-pin connector on the throttle body) WHILE THE KEY IS STILL
ON. This prevents the ECM from adjusting the idle speed while you make your adjustments.

Block the drive wheels, set the emergency brake, and start the engine. Set the idle speed by adjusting the idle
speed screw. The engine should be at operating temperature for this. The exact setting is on the emissions label on
the radiator shroud, but in general, the idle speed should be about 500 RPM in Drive, 700 in Park / Neutral, or if you
have a manual transmission, somewhere between 600-800 RPM. Remember that the truck is running during this
adjustment, so stay clear of the fan, and make sure it can’t roll or otherwise be put into gear while this is done.

Once the minimum idle speed is set, turn the engine off, reconnect the IAC, and remove the jumper from the ALDL
connector. The TPS minimum voltage must now be set. Turning the idle-speed screw may have moved the TPS idle
voltage away from the specification, so it should be adjusted next.

Connect a voltmeter between pins A (usually dark blue) and B (usually black, or black/pink) of the TPS, and turn the
key on. Don’t start the engine. Loosen the two torx screws holding the TPS in place, but don’t remove them. Rotate
the TPS until the voltmeter reads between 0.45 and 0.55 volts, with 0.50 being ideal. Tighten the mounting screws
(carefully, they thread into soft aluminum) and re-check the voltage to make sure it’s still within range.

That’s it. After the procedure is done once, it’s easy to remember and do. I hope this helps.
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'89 Troop RS (Exhaust Valve Challenged), now gone to a Good Home!
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Re: 1990 Trooper 2.8 High Revs and Sparks Flying

Postby NC_Trooper » Tue May 18, 2021 11:17 am

Thanks for assistance guys!

I investigated under my dash and actually took my entire interior apart to clean and checked all the wiring. Everything looks brand new and there are no signs of bad grounds, faulty wiring or rodent damage. The relays under the dash seem to function normally. I guess I will just keep an eye on things and keep the fire extinguisher close.

I have adjusted the TPS and got the truck running just peachy. I had a few vacuum small vacuum leaks that were certainly not helping.
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