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350 TBI surging

Jake_C_

Jake_C_

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So here’s the deal, this is on a 1990 Chevy g20 but it’s the exact same setup as a square body with tbi as far as I can tell and I trust you fellas advice more than the guys on a van forum. Anyway I just got the van two days ago, 80k miles. It didn’t run and the guy thought it was a timing issue. I threw a new battery and distributor in it, found tdc on the compression stroke in cylinder 1, rotor was already facing the #1 cylinder so I advanced the timing and it runs. It surges a whole lot at idle and has to stay advanced beyond the timing marks or it dies. It runs smooth but surges and eventually dies no matter what. If I retard the timing closer to where it should be it dies much quicker. I don’t see a huge vacuum leak anywhere although the line to the cruise control is a little dry rotted. What do you guys think the problem is? Idle air control valve came up in my research. Any help would be appreciated, thanks guys! IMG_8659.jpeg
 
Timing is set on a TBI by disconnecting the brown (?) wire near the firewall bulkhead. That disables ESC while you can set the timing to 0 degrees.

I would indeed check that the IAC is moving around... you can see or feel the IAC pintle inside the hole on top of the TBI. If it's not moving, it's probably not working.
 
SnackPack said:
Timing is set on a TBI by disconnecting the brown (?) wire near the firewall bulkhead. That disables ESC while you can set the timing to 0 degrees.

I would indeed check that the IAC is moving around... you can see or feel the IAC pintle inside the hole on top of the TBI. If it's not moving, it's probably not working.
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The wire is brown and black, disconnected. I took the IAC valve off and I can twist the end with my fingers and it retracts, I also think I heard it when the van died so I think it’s working. Still can’t hardly keep it running no matter where the timing is.
 
I would stop fiddling with timing and set it to 0. You mentioned the cruise control vacuum being rotted — perhaps cap it off and look for more vacuum leaks. Also, is there a good spray coming from the injectors?
 
Good visual inspection of everything. Hoses, electrical connectors, etc. TBI base gasket is prone to cracking as well.
 
SnackPack said:
I would stop fiddling with timing and set it to 0. You mentioned the cruise control vacuum being rotted — perhaps cap it off and look for more vacuum leaks. Also, is there a good spray coming from the injectors?
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I’ll set it to 0, pretty sure it won’t fire because it only seems to run very advanced. I’ll do some more vacuum leak hunting tomorrow. There is spray coming out of both sides.
 
dyeager535 said:
Good visual inspection of everything. Hoses, electrical connectors, etc. TBI base gasket is prone to cracking as well
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I gave it a good once over earlier today to look for anything disconnected. Haven’t looked at the gasket yet. The timing is what’s confusing me at the moment It doesn’t make sense to me why it will only run while very advanced and still surges and dies when it does run.
 
Get some brake clean or carb cleaner and spray around looking for vacuum leaks.
If the idle goes up where you spray that’s a leak that needs to be fixed.
 
You can also choke the IAC by putting an appropriate size finger in the air channel. If it still runs, you probably have a vac leak elsewhere.
 
80k miles the factory chain has stretched, maybe even slipped. This could explain the need to advance the dist timing to make run. Providing the dist is indeed stabbed and wired correctly. Check the exhaust flow out the tail pipe, if it seem low remove the o2 sensor and start it up see if it idles better. If does possible restricted cat/mufflers.
While testing after timing adjustment, was timing bypass unplugged or connected ? Tbi are notorious for base plate gaskets failing. You can see it sticking out in between the venturi's, engine off, air clean on the bench, open throttle wide, and look down if you see any gasket materiel it's bad. They also like to come loose, I always checked the bolts whenever I serviced a tbi. The IAC if want to prove it moves, remove it, plug it back in, turn key to run, it will run the pintle in and out. Also the IAC motor is not the only fialure point, the passage in the tbi body can be plugged and restricted, or have crud built up uneven on seat and Iac cant seat properly.

Nice camper looking forward to seeing more pics
 
Fastereddie said:
It’s also possible the balancer ring has spun leading to inaccurate information.
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Yes, you should double-check by pulling the #1 plug or something. There is also more than one type of tab and balancer combo for SBC so if some parts have been swapped the marks may be misleading. If you find TDC with a straw or whatever, you can paint a white line on the balancer. At least then you've ruled timing out to sort the rest.
 
Some of these im not able to do because I can’t keep it running long enough to test. I did pull the IAC valve and turned the key to run and felt something in the valve try to move but nothing happened, maybe it’s bad, I’m going to replace it and go from there. Didn’t see any gasket material inside, I replaced some suspect vacuum lines. When I first adjusted the timing the bypass was connected but after the suggestion I unplugged it and have noticed no difference. The TPS has been replaced by the previous owner.
 
The IAC might move after a little bit of cleaning. They get gunked up with carbon.
 
Well replaced the valve and no difference, what’s the default position of it at startup? I can hear the air being sucked past it
 
start up should slightly open, similar to the throttle plates on a carb at idle. Just enough air to allow it to run.
 
Ok I have it set to 0 on the compression stroke, first picture is where the rotor was pointing before, I moved it over one tooth and the second picture is how it looks after I moved it, which one looks more right?IMG_8681.jpegIMG_8683.jpeg

IMG_8682.jpeg
 
unplug the vacuum line to the egr, maybe give a light rap with soft face mallet, and see if idle changes. hard to see looks like rotor is pointed at number 2 now. As long as the wires match should matter in the end.
 
Wes Harden said:
unplug the vacuum line to the egr, maybe give a light rap with soft face mallet, and see if idle changes. hard to see looks like rotor is pointed at number 2 now. As long as the wires match should matter in the end.
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I agree, I thought it was clocked too far in the second picture as well but it also didn’t seem quite right in the first. I’ll try the egr thing today.
 
Ok let’s work wish the assumption that the timing is off, like if it was timed for tdc on the exhaust stroke for example, could it still run? And if so would those symptoms match the problems im having?
 
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