Engine : '90 LO5 350

A couple of months ago, my engine blew a head gasket, and while I had the heads off, I installed some ported small-chamber TPI heads, and an upgraded cam (204/ 214, .420"/ .443", 112 split).
Of course, the factory computer is confused by the strange MAP readings from the lower vacuum signal (15-16" hg @ 750 rpm), so I needed a chip.

I've tried FOUR of them, and none of them worked. Every time, the engine goes into "limp-in" mode and runs worse than it did before.
SES light stays on or blinks like a strobe-light; cannot retrieve any codes.
Reinstalling the factory chip eliminates this.

I've gone through the whole diagnostic procedure for this condition (from an official GM service manual), and every time it comes up being the fault of the PROM chip.
The tuner has tested the chips, and swears that they're good.

For whatever reason, this ECM just doesn't like tuned chips.
At this point, I'm about ready to slap a Quadrajet and big-cap HEI on it and call it good, but that seems like a huge step backwards to me.
And I'm not getting rid of the cam.

Anyone have any arguments to keep the TBI?

 

People have been adding "tuned" chips to TBI cars for a goo many years without this sort of issue. I'd say the problem lies elsewhere. You can always ditch the TBI if that's what you are wanting to do, but it should be tunable.

 

That cam produces more than enough vacuum. There's definitely a problem elsewhere...

 

I believe both of you guys, and I have used a chip from this tuner before, with no problems.

I just can't find anything else wrong, and the problem goes away when I put the old chip back in. I can't emphasize this enough.

I DO NOT want to take off the manifold, re-route fuel lines, wiring, etc.

Would y'all like to see a scan of the service procedure I used to determine that the chip was at fault?

 

WHAT SCAN TOOL ARE YOU USING?

Did you degree the cam when you installed it?

What is the cranking compression pressure?

Where is the initial timing set? Does the timing advance work properly?

 

For right now, I've just been jumping the A & B terminals on the ALDL connector. No response. SES light stays on.

No, I used double roller timing chain with a three-keyway crank gear, and installed it at 0' advance.

Average compression is 175 psi.

I set initial timing to 0', as per the instructions. That is with the tan/ black bypass wire disconnected.
When I reconnect the bypass connector, the timing DOES advance, but the SES light is still on, and the engine runs worse than it did with the factory chip.

 

Put a REAL scan tool on that vehicle. Verify the sensors and outputs. THEN look at "codes".

Blinky-light codes are four steps away from futile.

Are you sure the wires from the ALDL to the ECM are in good condition? Do you get proper codes (engine off) and rich/lean--open/closed loop (engine running) indications from the blinky-light when the OEM PROM is in place?

REAL common for folks to get mixed-up on the +, 0, - marks on those multi-keyway timing sets.

That's actually better than I'd have expected. It could mean you're in great shape. (It could also mean that the compression gauge is inaccurate; or it could mean the cam is overly-retarded which could cause it to build extra cylinder pressure.)

I put aftermarket heads on my L05; things worked better when I jacked-up the fuel pressure a bit, and advanced the initial timing to about 5 degrees BTDC.

 

Double check the ECM Service Number, someone may have swapped it out for a different one. A '90 should have the 1228746 ECM. But if it is a '7747 or '8063 then will need a chip made for that ECM. They aren't interchangeable.

If it is the correct ECM then it is likely a socket issue. The contact leaves in them get smooshed (highly technical term here) and don't make good contact with the chip leads. In this case use a small pic and get behind each leaf and gently bend it towards the outside of the socket.

This will provide better contact with the chip leads.

If all else fails can always try another ECM. The '8746s are rather plentiful.

RBob.

 

I'll do that when I get back to work, but I can't get ANY codes, not even a code "12" when the aftermarket chip is plugged in.

I can get codes with the factory chip plugged in, I just get really sluggish performance.

Yes, GM diagnostic procedure verifies good connection between ECM and ALDL connector.


Trust me, I didn't do that. An overly advanced or retarded cam won't keep the ECM from going into diagnostic mode.
Also, doesn't ADVANCING the cam INCREASE cranking compression? Intake valve closes sooner, trapping in more pressure?



I sent a picture of the ECM/ part number/ factory chip to the tuner, so he knows what he's dealing with. I will double-check the numbers you mentioned, though.

Thank you both.

 

$61 mask , is your tuner putting the prom in the carrier?

 

Yes. My mistake. You're correct.


I don't know enough about chip-burning to have a legitimate opinion...but I'm very suspicious of the guy burning chips for you.

 

No, he's in another state. But trust me, you'd have to be trying really hard to put the chip in wrong, and even if you did force it in (somehow), I doubt the engine would even start.

 

The prom can go in the carrier backwards and still install and run in lhm.
the notch of the 24 pin 2732 grace the notch in the carrier. The legs in the carrier prevent backwards installation.
but idk maybe you are updated with a moates adapter and 27sf512 eeprom .

 

Believe me, I'm not putting this chip in wrong; it only goes in one way.
I can provide pictures.

 

Okay, just seems like something ecm wise is causing lhm

 

Chip backwards will smoke it in seconds, very hot to the touch, etc. Been there and done that on accident once. It will start in LHM.

My guess would be the chip addressing incorrectly done or the wrong calibration mask being used. Either will cause LHM activation.

As for running issues, TPI heads will always have less low-end torque than the TBI heads. They also require 6-10° more timing advance almost everywhere. I would set the base timing to about 8-10° BTDC and see if it actually runs half way decent. Advancing the timing will increase vacuum. Also need a solid 13-14 psi fuel pressure for that cam in a TBI.

I ran that cam in a TBI swapped carb 305 and later the same setup with a TBI 350 short block swapped in place of the 305 shortblock. Both setups had 601 heads and that 204/214 grind. Both liked 8-10 BTDC on the timing and 13-14 psi fuel pressure running on the stock chip. Both ran great and had good driveability

 

I agree, something is definitely up with the chips.

I'll try the timing like you said, and see what I can do about the fuel pressure for now.

 

Okay guys, I got a chance to hook it up to the Snap On Verus Pro scanner at work, and...

Code 51. EPROM failure. That's once I plug the old chip back in; it will not communicate with the scanner at all with the new chip.

I'm getting a refund for the chip, and I'm going to carb it. In the meantime, I'll see about messing with the timing and fuel pressure to get this thing to run a little better in the meantime; it's my daily driver right now, so I can't put it out of commission for a few days while I get all of the odds and ends sorted out.

Thanks to everyone who helped.