1991 K1500 5.7TBI Hunting/Stalling in Park/Neutral

[Elk4one]

This is going to be a long one, please stick me on this and please forgive a Ford guy switching to GM

1991 K1500 5.& TBI hunts then stalls when in Park or Neutral once engine gets into closed loop in between 145 - 160 is when it starts. What i mean by stalling is it seems like it is running out of fuel, tach will go from 600-350 to 500 - 350 and then cut out somtimes it will do this for 10 minutes sometimes 20 secs. I can put it in drive all with brake or parking on and it will not stall unless you are driving and slow way down (about 3-5 mph) to make a right turn. With A/C on it does not stall in park or neutral or turning. No codes or check engine light, starts right back up after stalling/shutting off. While driving I have noticed the tach bounces up and down when letting off the throttle, but does not move if keeping constant pressure on throttle only while decelerating does the tach flucuate about 5-800 rpm. No other gauges flucuates, no lights nothing just starts hunting and dies.

I have changed TPS, Coolant temp switch x2, t-stat, TBI, AIC valve x2, TBI gasket, O2 sensor, injectors, plugs, wires, cap, rotor, ignition cotrol, distributor, EGR, EGR accuator, vacuum lines, CAT, 15 PSI pressure regulator, brake booster (wasn't leaking but rusted and looked horrible) , new ground cables, MAP sensor. Pretty much every engine/eletronic control systems has been changed or gone through.
No broken connectors or wires that I can see

No vacuum leaks, runs pretty good for a 30 yrs old truck w/stock engine and only 147K on odometer, will run 80 down the highway without an issue except for horrible gas mileage.
Gas mileage is horrible talking big block numbers 8-11, cruising at 70 long trip abouy 140 miles avg about 13 through Nebraska.
New/rebuilt transmission less than 1k miles, problem was prior to tranny self destructing. All fluids have been changed

Can someone please point ime n a direction I have not already been. What am I missing here? Am i loosng a fuel pump, ECM? wouldn't think so as I can sit in drive and not stall but about 15 secs after switching from drive to park it starts hunting and then stalls out, but truck will restart immediatly and idle for a time then start all over again. Unpluggin the O2 makes go high idle (righ((normal), unlplugging TPS kicks check engine light on, Unplugging IAC make engine go stupid, unplugging coolant sensor also kicks it in high idle rich condition.
I am not a novice mechanic, 30 yrs of working heavy equipment (Diesels)

I think I covered everything that I have done and what the truck is doing. I am open for suggestions hopefully someone on here has the same issue and figured it out

V/r
Elk4one

 

[PlayingWithTBI]

I am open for suggestions hopefully someone on here has the same issue and figured it out

A few things;
What's your fuel pressure? You said you put in a 15 spring but did you confirm that?

Did you set your timing to zero BTDC with the black/tan EST bypass wire disconnected? How's the idle when it is disconnected?

You changed the ICM (ACDELCO I hope), did you change the ignition coil too? How's the pickup coil inside the distributor?

 

[Jeepwalker]

Those are good suggestions.

Also
- Put a vacuum gauge on it ...what's it look like (doing?). Take a propane torch with a hose on it (unlit) and wave around the intake to check for leaks. Or use some carb cleaner in targeted squirts.
- Verify engine (ecm) temp gauge ohms, cold and hot. You can do this with a multimeter. Could be your new one is flaky or wrong application. Make sure it's the ECM temp gauge you're checking.
- Throttle bore and butterfly is clean? (not carboned up)?

 

[Erik the Awful]

I would double-check for vacuum leaks.

Schurkey will be on here before too long to tell you to use a scan tool and check the data stream. He's not wrong. Do you have any way of seeing what the sensors are reading?

 

[Elk4one]

Appologies for not being to get back to you guys sooner

No scan tool available to me without buying one.

I am pulling 17-18 inHg (I live at 6250ft) not great but not horrible should be around 19 at this altitude. Needle does not bounce in gauge and responds like it should when throttle to 2500-3000 and off.

New TBI so bore and butterly are sparkling.

When I dropped the dizzy I made certain it was at zero w/brown (EST) disconnected. Idle drops when disconnected. I changed the ICM, Pick-up, Distributor, Cap and Rotor and Coil.

As for the ECM temp gauge, your talking the one beind the t-stat correct and how can tell if it is the wrong application??

I have tied the propane and get nothing around the bolts or runners but when warm I can not keep it running long enough to get a good check in as it starts to hunt and falls off.

I need to find a correct t-fitting to check proper fuel pressure, I might be shooting in the wrong direction the whole time and have a half working fuel pump.

Elk

 

[tayto]

with that much milage and unknown if pump has been done before i would replace it. use a TPI or vortec pump. you said "new" throttle body so i assume that means a properly rebuilt unit with throttle bushings etc?

i will also echo: get a scan tool. if you have a laptop you can buy a datalogging cable and use free software....

 

[Jeepwalker]

How far away is an auto parts store? Usually most will scan your vehicle for free. But call in advance. Most scanners these days are OBDII only, depends the scanner they have. But best to have your own you can continue to refer back to.

This says this runs on a pc and Android. Looks basic. IDK anything about it:

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Not trying to spend your money, but this 'ole brick' scanner would do the job for you. Though more expensive, it's a better overall deal. Maybe you can find a local one that's cheaper but this is about what they go for. Too bad you didn't know a guy in your area who has he doesn't use much. The troublshooter cartridges might help you out too. I see everything you need in there for your truck. Ford connectors too. Looks like the cartridges work up to 2001. Those earlier 'red' cartridges were a lot faster than the later black ones. These old Snap-on MT2500's are big and somewhat clunky but easy for a beginner to use and the best overall obd1 tool:

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There are some cheaper Innova units out there. I don't know anything about them. If you get one, you need LIVE DATA, and a few bi-directional controls are handy. And whatever other vehicles you have ...you might want to consider a scan tool that will work on those as well. A scanner is just a tool. Sometime indispensible, but they can lead you in a lot of stray directions too oftentimes. Usually problems come down to basic issues, and on OBD1 vehicles most of the things can be tested with a VOM.

 

[Jeepwalker]

Let's go back to basic troubleshooting for a minute:

When you had the distributor apart, did you put a new one in it, or a used one? Is it the 'right' one? Did you inspect inside the distributor (if used). The pick-up coil can get flaky, Is the ignition module the right one for your vehicle (is it a good one or import)? Did the hold-down nut come loose and dist move? Cracked cap, or full of moisture? Any corroded ignition connectors? See the other thread going on now about a cracked magnetic rotor inside the dist (mine was like that ...and it ran like crap too). I only discovered my truck's issue after I'd exhausted everything else. Ign wires are for sure good?

Have you checked each spark plug for spark delivery? It might be time to pull off a wire at a time if you haven't done so. I've replaced plugs and later (after a lot of wasted time) one of the new plugs was bad inside too. Also inspect the plugs in case your head gasket is going bad. Have you performed a compression test? In case your engine has a burnt valve or bad headgasket? Your vacuum is low, how about blocking off all the vacuum ports and re-trying it (if you didn't before). Perhaps there is an intake leak or leak at the TB base gasket. Could there be any water in the fuel? That can cause a crappy-running vehicle much like you've described (sometimes it can take a good amt of fuel and many jugs of HEET to remove).

How are your engine and body grounds? Are they super-clean? Battery good? Electrons flow from Neg -> Pos. A lot batt can cause problems. Worth doing a load test (not just a VOM test). Did you inspect/wiggle wires at the ECU? Could there be any broken or rodent chewed wires? Is the PVC valve/grommet/hose ....all good? You ohmed out the temperature sensor (cold & hot) and compared it to a temperature/ohm chart for your year truck, right? Is the EGR passage clean? You cleaned it out right? Is the vacuum going to the right place? All the vacuum routing is correct?

I would go back over the basics one more time. OBD1 systems are pretty simple. A scanner would not pick up on most of the above issues