400 cid into '91 Gmc

**chuckander** · 03-13-2014

Refilled with 6 quarts (extra quart for factory oil cooler). Ran it for 20 min until it started to surge again. Checked it right before running, right after running, then again 5 min. Later. Raw gas smell came back after running and as far as I could tell stayed the same when checked yhe second time. Also checked the tbi after running. No leaks from gaskets, body or drips from injectors. Checked spark plugs again and still white like lean running.

**chuckander** · 03-13-2014

Sorry thought first post didn't go through and forgot to mention plugs

**chuckander** · 03-16-2014

I went ahead and changed the intake gaskets and the gas getting into the oil has stopped and it runs nice and smooth.. when it's cold. Once it starts to warm up it'll surge slightly and then will refuse to idle at all. Ive checked all the other gaskets and vacuum lines for leaks and still can't find any. I'm at a loss at this point. .

**EagleMark** · 03-16-2014

Sounds like it's going into Closed Loop when surging and dieing? Still not surprised with a 400 on what injectors? What ECM? What chip? What fuel pressure? Where's timing set? What codes are set? Do you have a data log?

You could try disconnecting the O2 sensor and see how it does when warm.

What is Check Engine Light doing?

**chuckander** · 03-16-2014

It's still running the factory ECM, injectors, all under factory settings, fuel pressure is 12 to 14 psi. Check engine light only comes on when key is on and engine isn't. Like I said before the only after market part is the airgap intake manifold. It is running the preinjection 350 heads.

**EagleMark** · 03-16-2014

Originally Posted by chuckander

It's still running the factory ECM, injectors, all under factory settings, fuel pressure is 12 to 14 psi. Check engine light only comes on when key is on and engine isn't. Like I said before the only after market part is the airgap intake manifold. It is running the preinjection 350 heads.

Yup, you've said that before! But I have no idea what factory ECM it is, or what factory injectors, or factory chip.

Even if I did the outcome would probably end up the same. Like a stock 400 engine needs more fuel then a 350, or less fuel then a 454, but I have no idea what injectors you used? The the air gap intake is designed for a modified engine and when tuning it needs more AE, but I'm not sure on a stock engine? It's power band also starts at 1500 RPM so it's not any help with a stock engine and actually a harm in idle and low RPM driving.

No matter what you do, the only way to get this engine to run right is with a modified chip, then some data logs to see what it is doing.

**chuckander** · 03-16-2014

Sorry, the pickup it's in is a '91 half ton GMC with the 350. I didn't know if they used different ECMs for the same pickup. I know they were different between engine sizes. Was there a letter or number indicator that I could look for? I know someone who has an aftermarket chip for a '95 chevy or Gmc. Would that work? Or do I have to look into a custom made chip? Was also told that if I replaced the airgap with a factory manifold that might work. I don't want to start throwing stuff at it if I don't have to.

**EagleMark** · 03-17-2014

There should be a four letter code (BCC) for the chip in the ECM on a sticker. Or it is also directly on the chip inside.

A 95 chip won't work in your ECM.

There's only one way to make the engine run right and that is to know what chip is in truck so you have a starting point of where to adjust from, then data log, adjust chip, burn and run it with another data log.

So you have 5.7L Injectors as well? These will require more pressure to supply enough fuel for the bigger engine. So adjustable regulator. What is your fuel pressure now?

Not sure what you mean by swapping back to stock intake might work? If the stock intake will install correctly on the heads it would be a better choice for a stock motor. But still not going to run right without a chip calibrated for the 400 engine. Chevy never made a 400 TBI engine so you can't find a correct chip for it!

Surging at idle can be from the chip alone, or vacuum leak, or throttle plates not set right or... we are just guessing here at possible situations and that's all it's ever going to be without a data log to look at!

**chuckander** · 03-17-2014

The 4 letter code that I can see is either ASDU or ASOU. Sticker is a little wrinkly. The fuel pressure actually fluctuates between 12 and 14 psi running. I tried plugging my diagnostic tool to the port and it can't make a connection. So getting a data log might be difficult. It looks like someone changed out the ECM because it has yellow marker on it. Would a set of big block injectors help? Or will I have to find a higher pressure fuel pump? The reason I asked about the manifold is because I figured less manifold volume would be better.

**chuckander** · 03-17-2014

Sorry misread you post I looked on the sticker on the outside of the ECM. I'll pull it out and look inside and get back to you.

**EagleMark** · 03-17-2014

1227747 and ADSU was used in that truck so probably correct.

With BB injectors you would not have to raise pressure or change pump.

I would not use that intake on a stock motor...

**chuckander** · 03-17-2014

What hardware, software, and specifications do I need to reprogram the chip for my pickup? Can I do it without removing the ECM? Or will I have to do it outside of the vehicle? Can I make any modifications to the factory manifold so it will fit? Or can I use an old carburetor manifold and simply use the factory adaptor I have for installing a tbi on the big blocks?

**EagleMark** · 03-17-2014

It's got a chip in the ECM, so no flash.

You'll need a Moates G2 adapter that requires soldering into the ECM, couple 27Fs512 chips, Burn 2 chip burner, ALDL cable, TunerPro RT software.
https://www.moates.net/gm-19851995-c-64.html

Not sure on the intake?

A four barrel intake and TBI adapter will work. Although the factory adapter works well, it's UGLY!

Plus it adds a couple inches, make sure you have hood clearance.