I have an issue with a 92 350 TBI in a G30 van I’m trying to solve. Basically what happens is it cold starts fine, a little long of a crank time but not that concerning. It’ll go through it’s high idle on cold start and starts dropping down in rpm once the coolant temp gauge comes up a bit as it should. If you snap the throttle during this time it responds perfectly.

The problem happens when you put it in gear. If you don’t touch the throttle it moves fine. Once you give it any throttle it coughs and bucks a bunch for about 5 ft. After that it’ll just come right out of it and never do it again till another cold start. Doesn’t matter if it’s in reverse or drive.

If you look in my post history, you’ll see that I cam swapped an L31 cam into the engine recently. I don’t think that has anything to do with my issue though. Ever since I’ve owned the van it has had some variation of this issue. Always first morning cold start. It’ll idle great and you can snap the throttle fine but first attempt to drive it bucks and comes out of it never to do it for the remainder of the day.

Some more history as to what’s already been replaced:

The TPS is a few years old.AC delco brand. Reads normal on scantool. Voltage varies with the slightest throttle movements. Pigtail harness was also installed.

EGR was replaced last year with a NOS part and part number matched to the old original GM one. I know of the pos/neg pressure variance.

IAC was replaced years back and seems to work as it should.

Throttle body recently rebuilt. I have two new rebuilt injectors and the thicker injector pod spacer on it. Fuel pump new with pressure bumped to 15 psi at idle to account for the different cam.

02 sensor is a three wire GM sensor I retrofitted so I could have the heater circuit. Allows me to go into closed loop faster. I’ve confirmed this works with the scantool. If I flip the switch for the heater circuit it’ll go into closed loop within 5 seconds.

Coolant temp sensor is a new NGK sensor that reads correct temp on scantool. A replacement pigtail harness has also been installed.

Distributor/cap/rotor replaced a few years back.

MAP values look fine on scantool.

PCV valve, grommet and hard line new.

Intake manifold gasket new and no vacuum leaks to be found.

Not sure this affects anything but my thermostatic air cleaner valve doesn’t work. The flapper was messed up when I bought it so the bottom hole that would usually connect to the stovepipe down to the exhaust manifold heat shield is just taped off with aluminum tape.

I just wanted to see if someone has seen this exact problem before and can streamline my diagnosis before I start really tearing into it.

Thanks.

 

If you look at the GM tuning the open loop air/fuel ratio can be extremely lean or extremely rich depending on the year model and emissions calibration. It could be practically drowning in fuel or starved for fuel. I have had to correct both conditions.

 

Not a fan of putting the thicker spacer on an injector pod unless it's spraying into a larger-than-stock throttle blade; or the 454 throttle blade those spacers were originally used on. But not likely the source of your troubles.

As cold as the exhaust would be directly after cold start-up, this is probably also not the source of your issues...but worth grabbing a different air cleaner housing from the Treasure Yard so that it becomes functional again.

I'm assuming that the flapper is "gone" so that it's not blocking flow through the snorkel...right?

 

Was this done through ecu tuning?

 

I could remove the spacer and see what happens. I agree it’s likely not the cause since it did this for years with the stock spacer. Sometimes it would go away and I wouldn’t have the problem in the morning, but it’s back now and probably as bad as it’s ever been.

I could get another air cleaner. I’m sure they’re around at the JY. And yes the flapper is gone so no issues with it being closed off.

I was thinking of doing an idle relearn to see if that helps. Didn’t do that after doing all the mods. Not sure if that could cause this issue since it only happens once throughout a drive cycle.

I’ve confirmed that the 02 sensor switches as it should and it seems to be working fine, but I suspect this has something to do with open/closed loop fuel control somehow. I’ve seen the scan tool switch between rich and lean as it should, and I’ve seen the voltage vary quickly. I also graphed it on an oscope and the waveform looks fine switching between .9 and .1 as I drive.

 

I'd want to see what the data stream is showing when the throttle is pushed, in gear, leading to the stumble.

 

Just an update. Think I found the problem. I started my diagnosis with the egr valve. Used a block off plate, and the problem went away. Bolted the egr back on and no more problem. Can only assume it was something like a gasket leak since the problem hasn’t come back since. Thanks to everyone that chimed in, always appreciated.