[808-Con3]

7.4L 1992 GMC sierra 1ton
--Problems just started, i have to fully depress pedal to start
--injectors have been replaced, they pulse during ignition...when started, injectors spray constant, dumping fuel
--NO o2 sensors on the manifold

**NO engine codes, only code 12
**Can't figure out why injectors are not pulsing, idle revs up to 1,500
--ECM replacement? TPS and IAC need to be replaced?
--everything on the harness gets the correct Voltage.

I'm thinking that the ECM needs to be replaced, the IAC and possibly the fuel pressure regulator.
Very hard to maintain an idle now on the truck and the conical spray pattern is thick and constant when the truck is running.

No MAF sensor or O2 sensors on this TBI 454 engine...it's a 1ton.
again, i replaced the injectors but it's not fixed yet.
Played with the idle screw to no avail as well. :grrrrrr:

 

[Bigg Ron]

Welcome to the club!
Have you checked the crank sensor?

 

[Chris Miller]

Your ignition control module sends out the signal to pulse the injectors. Have you replaced the stock distributor? If not, have the module tested. If so, put the stock distributor back in. If all that's good, you should check the harness from the module.

 

[808-Con3]

Thanks guys!
Yes, i checked all the sensors but only a voltage check.
Haven't checked the distributor yet.
Stock everything. After i pulled the harnesses to the distributor from the ignition coil...truck wouldn't start. That's as much as i did with that yesterday.
Sounding like the ECM computer might need to be the last thing to replace...
I'm gonna recheck the harnesses....i only have a chilton's for this truck not the FSM and the voltage levels per harness aren't exactly listed well...
I'm gonna guess the usual 12v or 4.5-5v??? Sound about right or do i need to locate a more specific level and drop or variation under load for this harness?

 

[Crawdaddy]

Check the ignition module that Chris mentioned. It's under the distributor cap and goes out a lot. It's probably not the computer.

 

[808-Con3]

Solid! I'll post an update later today thanks again for the help so far.

 

[808-Con3]

Replaced the Ignition module, Throttle position sensor, and the Idle Air Control valve.
No progress. Injectors still idle with steady stream of gas and idle holds around 1500 RPM.
Only thing left is MAF sensor (maybe called MAP?) and the ECM right?

Not sure what else would be signaling the injectors to pour gas.
Is there a relay or something else along the chain that might be stuck or not operating correctly?
I switch the under the dash relays around just for fun and that did nothing either.
Truck gets harder and harder to start through out the day and holding the accelerator to the floor is a must for the truck to start at least 98% of the time.

...hmmmmmm

 

[808-Con3]

MAP sensor is next...if that's not it, then the ECM...

Crank pos sensor...Bigg Ron...where is that one...
--Front drive side of engine or back tucked up by the starter on the passenger side? Didn't see anything else. Haven't had or heard any problems with the flywheel or starter though...heard starts and idle can be rough though if the crank sens. is out...
After messing with the MAP sensor, unplugged and the engine light came on...ran better and raced up to 1500 when plugged back in...so makes me wanna rule out the ECM and crack sensor so far.
Thanks again guys...always helps.
First time i've ever worked on a TBI...

 

[stephan]

Hey Con, if your voltages all checked good & you see them pulsing during cranking then I think this is all good, ECU's good, & dist. module's good. They're just too fast for you to see them at idle. If you hook up a timing light & point it at the injectors & watch them at idle RPMs you should be able to see they're fine cuz it will cut the actual real time down because you'll only be able to see every 8th revolution. I think your high idle is something else, rather than injectors dumping.

 

[808-Con3]

Yeah, replaced the MAP sensor..problem persists.
The plugs keep getting really fouled out with gas and are black drippy messes.
I'm gonna clean the plugs again, check the timing again and go from there.
--CRANK sensor...couldn't find it yet....really hard to see around the starter motor...I don't have a lift and it's 4x4.
I'll do my best but my manual doesnt show a location so im going on past experience that it is located near the starter motor...anyone know where that is exactly?

This sure is one of the harder problem finds that i've had in a while...kinda funny because i know it's something easy.

 

[Chris Miller]

You don't have to worry about the crank sensor -- they didn't put them in until 1996.
If it's running that rich still, the only other thing I can think of is your oxygen sensor. Luckily, that's easy to change, and if you're really lucky, it's the 20-dollar one-wire unit. If it's stuck sending a lean signal, it can make your engine use a horrendous amount of gas.

 

[808-Con3]

Ya, this 1-ton does not have an o2 sensor.
Blanks on the manifolds...no hole or sensor.
Been looking at that since the beginnning but there was never any sensor or hiring harness

---------- Post added at 03:30 PM ---------- Previous post was at 03:28 PM ----------

exhaust with straight pipes + H-pipe.

 

[Chris Miller]

Look about six inches in front of the catalytic converter. They weren't all on the manifold. It does have an O2 sensor. They've had them since the 80's.

 

[808-Con3]

Ya, it must have been deleted then...only an H pipe and flowmasters.
NO harness wire can be found though...dangling or otherwise...

Sure they had em on the 1tons?

 

[Chris Miller]

Yeah, I've seen a few of them. You must have one of the weird ones. :3
Well, two other things I suppose you could try...
Replace the fuel pressure regulator diaphragm. That's pretty cheap and easy to get.
Try also, unplugging the vacuum hose from the EGR valve and block it off. See what happens. That's a long shot, though.

 

[dualdj1]

Pretty sure those don't have a crank sensor. Sounds like the ECU might be running in limp mode, or that your pressure regulator is bad. Didn't see but have you checked your fuel pressure yet? High pressure = more fuel through the injectors.

 

[808-Con3]

Yes, replaced the regulator, new Computer, New IAC, TPS, MAP...after the new computer, new plugs and wires...spray is finally a fine mist, truck starts right up...but now the idle RPM flutters from 800ish...holds then jets up to 1500 then back down...up and down up and down.
I'm gonna try the EGR vacuum and reseting the idle screw procedure again...
if that doesn't work...maybe try drilling in an O2 sensor and wiring it to the COMPUTER....just will need to find out what wires would hook into the computer to do that.
But that would be a long shot for sure esp when there wasn't an o2 on this truck.
Definitely have one of the weird ones im thinking...but its making progress at least.
ANy other thoughts?

 

[808-Con3]

After the new computer went in...and new wires and plugs again...ran good. Still surged up on idle though...up and down. Turns out it was the thermo sensor on the front top of the block/head. Tellin' the computer that the temp was -30deg so it was attempting to warm up or something like that.

 

[udidwht]

I know this is an old thread but the one thing that wasn't mentioned and likely is the cause...

Coolant temp sensor

 

[Napoleon Solo]

I know this is an old thread but the one thing that wasn't mentioned and likely is the cause...

Coolant temp sensor

Hello,
I have the exact same issue with my truck. Your post caught my eye because I just replaced my radiator a few weeks back and coincidentally or not my 1500 rpm idle issue started shortly thereafter. Can the coolant temp sensor really cause this?
My truck is a 92 GMC 7.4l 2wd.
Thank you,

Solo out...