I’m having some issues with my 89 K2500 with the original TBI. I rebuilt an L31 and put it in the truck, it was built to stock except for a GM “towing” cam. It’s got a square bore intake with an adapter plate for the tbi. I replaced all the normal stuff like cap, rotor, plugs and wires when installing it. I pulled the wire for the ignition and set the timing to 0 degrees.

Truck starts and idles fine, and will take off no problem, but there’s a hesitation part throttle. If I slowly accelerate it will wind out fine. If I bury the pedal it will fall on its face. If I go to WOT sitting in neutral it will stumble and backfire through the throttle body and if I stay on it then it eventually will stall.

The only thing I have tried is rebuilding the fuel pressure regulator. I don’t want to throw parts at it. Any one have some advice?

THE FIX/PROBLEM: corroded tps connector.

 

It has been a while but IIRC when I had my 89 factory timing was 4 BTDC not 0 have you tried playing around with that?

What is your fuel pressure?
What are the specs on the towing cam?

 

I haven’t tried playing with the timing, but I will. That is an easy check.

I haven’t put a gauge on it yet, I don’t have the fitting to place the gauge inline. The fuel pump was replaced a few years ago, the filter was changed with the engine install, and the regulator was rebuilt yesterday. It did seem odd that the spring in the rebuild kit was shorter than the original.

I will attach a photo from summit of the cam specs.

 

It really needs to be tuned, along with higher fuel pressure to feed that engine. At a minimum set the base distributor timing to 8° BTDC. This will help due to the Vortec heads.

RBob.

 

I used that cam with the intention to go with TPI and use Megasquirt but after seeing the price on a vortec tpi base I passed on it and stuck with the TBI. I’ve never played with the OE ecms. I do have a MS1 sitting on the shelf I could use in it, but I’d have to no use the IAC if I wanted to control timing with it.

 

Just went out to verify timing. It was set at 0 degrees with the wire un hooked. I advanced it a few degrees with no change. The injectors seem to both have a good spray pattern.

I did noticed with the light, as I rev it the timing does not advance very much. This is obviously with the truck in neutral and no load. Could that be a possible sign of a bad ignition module or a pickup in the distributor?

 

If you're reving it with the EST connector disconnected, then it shouldn't advance at all I wouldn't think.

With out the ECM commanding the timing, I think it'll just sit there at or about the base timing. It's not like the old mechanical distributors with the weights and springs..

What was the stock cam specs? To the first order, I agree with RBob... you probably need to tune it. If everything is in order from a hardware perspective, then my guess is your accelerator enrichment (AE) needs to be dialed in to match the new engine configuration (bascially the electronic equivalent of an accelerator pump on a carb). Speed density EFI systems are pretty sensitive to engine hardware changes.

From what you're describing, you're probably going way lean on throttle changes. The fact that you're saying the engine recovers after a brief period, that leads me to think it's not fuel pressure problem (otherwise it would never recover). It sounds like an AE re-tune is in order.

 

That’s what has me baffled. The truck feels like it’s got a lot of power, it’s just idle to WOT it stumbles and eventually will stall if I keep it there. The hesitation at cruise will usually clear up.

I guess I am surprised the AE could cause the engine to stall. I’m no tuner though. It my megasquirt controlled 350 I can turn AE off and it will stumble for a second but eventually pick up. I know that’s a different animal though. I tried to attach videos of what’s going on but they aren’t working. I guess j need to read into reflashing the ECM.

 

If you're already lean at idle due to the cam change and you go WAY lean on AE during a huge throttle change, I could potentially envision it actually stalling.

If you're steady-state fueling is also way off, you may actually unlock a little more power with a proper tune. In closed loop operation, the ECM can trim the fuel to some extent (even on speed density), but any fueling sub-routine where the ECM doesn't use the O2 sensor for guidance (i.e., open loop fueling when cold, AE, power enrichment, cranking fuel delivery, etc), it's going to be off with no ability to self-correct.

 

Your cam has enormously more duration than the OEM TBI cam, or even the Vortec L31 cam. MAYBE that's causing your problem, or part of it. The cam you chose is pretty mild, but the OEM cams have a frightening lack of duration. They're BEYOND "mild".

"I" would verify fuel pressure under load, and connect a scan tool to look at the data stream. You could be dealing with a faulty coolant temp sensor, a faulty MAP sensor, or even a weak ignition coil. The possibilities are almost endless. You need more diagnostic work to narrow down the source of the issue. I had misfire on my TBI until the mainshaft in the distributor was replaced--the magnets by the pickup coil had cracked and that was screwing-up the signal to the module.

Was the cam degreed when installed--or just "dot to dot and hope for the best"?

What is the cranking compression of all eight?

 

I regret picking this cam for the truck now. Now that you say that, it got me thinking. I bought the truck minus the engine, but all the stock TBI stuff was still there. I got a junkyard 350 and threw it in there, that engine had an occasional backfire. I wonder if that cam could of been slightly more than the TBI cam? I need to get the adapter so I can check the fuel pressure. I will check compression on all the cylinders also. I need to verify all the constants before I start throwing parts at it. The cam was not degreed, just a dot to dot line up and pray it’s right.

Let’s say I do need to play with the tune. What’s the easiest way to get into the stock ecm to re flash it? I have a MS1 I could use in it, but I would loose control of the IAC or ignition if I do that.

 

It's not a "flash" in the sense of the newer PCM's. The old OBD1 GM computers used a removable PROM chip. I'm not an expert on TBI computers, but my guess is they're similar to TPI in that you have to remove the chip assembly from the computer and burn a new program onto the chip.

Contact Tunedperformance. I think he can prbably get you squared away.

 

DON'T dick with the "tune" until AFTER you know this isn't caused by a faulty part, or incorrect cam timing.

 

Well after banging my head on the wall all weekend I found the cause. Corroded TPS connector. I was going thorough checking voltages and discovered it. Cleaned it up and put some diaelectric grease on it.

There is still a slight hesitation on quick throttle changes, but it clears up. I’d say that’s cam related.

So new question, what direction should I go for changing the “tune”? I have played with megasquirt a lot and am comfortable with it, and am starting to play with TunerPro for an LS swap I am working on. With these prom chips, am I better off to pay someone to do it?

 

You are not yet ready to be screwing-up the "tune".

You still don't know the fuel pressure.
You still haven't looked at the data stream.
You still don't have cranking compression figures.
You still don't know that the cam is synchronized to the crank as intended.

AFTER you've got all that out of the way...you MIGHT be ready to screw with the "tune".