please help me diagnose 350TBI

[Flo]

Hello all,
I know this is not actually a performacne question, but I am not active on any other forum where I hope for this know-how found here.
Girl walks in the shop "I was told you are good with those old American cars....."

Well, no I have a giant 146"WB G30 beast blocking a bunch of space. Cannot even get it in my shop and on the lift. Well, luckily when working on a van you are "inside" anyway.

Somewhat of an interesting vehicle (I am into "Americana" ). Former church van 1991 GMC Vandura Rallye STX- this thing was loaded from the factory with cruise, tilt wheel, dual AC etc. g30 HD chassis with GVWR 8600lbs
350TBI and 4L80 (I think).


When I got into it, it did not want to run at all. You could fire it up, but it stalled pretty much right away, did not take gas.

At this point I have replaced:
TPS, EGR, coolant temp sensor, oil pressure sensor, IAC

I confirmed the timing (unplug the dignose plug and it stays steady around TDC)

[Flo]

Ok, this is where I am stuck:

The TPS and EGR valve were really shot. Ever since those are replaced it actually runs after some warmup and accepts throttle. I plugged all vaccumports except MAP and brake booster to eliminate errors.

Has horribly unstable idle. It oscilates between 300 and 1200rpms (and sometimes stalls). I made some videos, maybe they can explain this better than text, I also have the scanner connected while running, so you see some data.

This is cold start + some peddling in the beginnning to keep it alive:

cold start + some gas by Florian Scherer, on Flickr

[Flo]

idle after warmup:

idling by Florian Scherer, on Flickr

[Flo]

with MAP disconnected:
MAP disconnected by Florian Scherer, on Flickr

[Flo]

now this I found most interesting, this is with the IAC disconnected! Still does the same oscilation. I am not sure how this is even possible.

IAC disconnected by Florian Scherer, on Flickr

[Flo]

From all I can tell the engine is bone-stock around 110t mls but seems in ok condition.

Someone has been into the TB before. Supposedly the TB itself was apart and has been US-cleaned and reassembled with new gaskets. It looks plausible, can someone f... this up?

I personally have replaced the sensors and the IAC as well as checked the timing.

How would you continue? I am still baffled that it can swing the idle like this without having an IAC.

Any help appreciated before I waste more time on this.....

Greets and thanks
Flo

[dannobee]

Is fuel pressure ok? Yes, it's hard to check on those. Plugged fuel filters were common too. I mean REALLY plugged, where you couldn't even blow through them.

Does the idle speed smooth out with a shot of carb spray? If you plug both valve cover holes with your hands (pull out pcv valve and snorkel pipe), can you feel vacuum, indicating a vacuum leak at either the bottom of the intake gaskets or a cracked intake manifold at the crossover?

Are the grounds good and tight at the thermostat housing?

[Flo]

I need to point out that it runs reasonably well and has throttle repsonse. You can hear that in the videos. Just the idle is unstable and oscilating. That is why I did not dig further into this direction.

I do not have the adapter to check fuel pressure on those to confirm though. Not very common over here.

But I will check for the "vacuum in crankcase" and ground!

[Schurkey]

Remove cap and rotor. Inspect magnet on distributor shaft. Broken pickup coil magnets are very common--generally cracked through the rivet hole. They cause unstable ignition--misfire.

TBI_HEI_02.jpg

TBI_HEI_03.jpg

TBI_HEI_04.jpg

Need to replace the mainshaft when the magnet is broken. Or replace entire distributor--but aftermarket replacement distributors tend to be of considerably lower quality than the GM originals.

The actual pickup coil itself seems very reliable. But anything can happen. Same for the ignition module--they don't usually cause problems, but they can fail in various ways leading to improper operation.

Leaking intake gaskets are somewhat common. The TBI gaskets are "special" in that the coolant passage at the rear is deliberately blocked with just a steam-hole. This is important when the OEM TBI intake manifold is used.

K1500_ZZ4_Intake_016.jpg

Dripping injectors, faulty sensors, and defective "tune-up" parts--cap, rotor, plug wires, plugs, fuel and air filters, PCV and EGR system, etc. are all possibilities. O2 sensor is suspect, "probably" needs to be replaced.

Have you verified knock sensor/spark retard system? False knock counts can cause unstable idle. Assure spark advance (electronic) is working properly.

Make sure PROM and CalPack are seated properly in the computer. But those should throw a code.

[Racer97]

Definitely check fuel pressure. The regulator spring breaks from rust. Look at the hard stop for the base idle, someone may have cranked on it to get idle. If it was cranked re set to 550 with the IAC disconnected. Remove IAC and check to see if it was broken . The gasket under the throttle body may be damaged. That can be checked with the carb cleaner or propane.

[rebelrouser]

From your video it sounds lean, either vacuum leak or low fuel pressure. From my memory you checked the fuel pressure by removing the fuel filter and the tool to hook the pressure gauge replaces the filter. So you may be able to make one. Second you scrolled through the sensors pretty fast but the O2 did look lean. Those old ones needed to warm up a little to get the O2 working. Low voltage is lean .9 volts is rich it should bounce around .5 volts if fuel mixture is right. Take some carb cleaner and spray down the throttle too much and you will kill it. When spraying the carb cleaner watch two things, first if it is lean the idle should improve, second if the O2 is functioning correctly the voltage should spike around .9 volts. If you cannot make the O2 spike with carb cleaner it is bad, might not be your whole problem but it needs to be fixed.

[Flo]

Hello all,

thanks for the input.

It does not misfire at this point (it did before I replaced the EGR-valve). If you look at and listen to the videos it actually runs pretty smooth. Just the idle "swings" up and down.
In the alst video I disconnected the IAC. No difference! Idle still swings between 300 and 1200. There is no way I can adjust the idle to 550. It just keeps "swinging". This is what leaves me scratching my head. How can it do that?

Also it stabilizes when crecking open the throttle.
I will try to somehow check fuel pressure.

Before I replaced the broken EGR valve there was another crazy phenomenom:
once it warmed up enough to go into closed loop it completely smootheed out and ran fine.I will try to get there again. My neighbors hate me to warm this thing up out side until closed loop..... but I will check on that again.

Greets
flo

[econo racer]

Have you tried unplugging the cts for a minute or two? distributor springs may be going crazy. Also change the pcv if not already. Also run a new ground from the motor to a good spot. Sometimes it is just the cheap things not seen, Shurkey may be on it.

[Flo]

What is the CTS?
This is an electronic distributor. It does not have springs as far as I can tell.

[econo racer]

I see that I was not sure of what year you have. CTS coolant Temp sensor-maybe on the front passenger side on top of the intake close to the t stat. Am I missing something but what year is this van.makes a big difference in diagnosis.