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Problem with motor

i have a 95 chevy tahoe with a newer vortec engine. a gmpp intake that can sit on vortec heads and take a TBI sits on top. it has all OBD1 emission items and stock ecm. all ignition components are new. it has a modded exhaust with headers and dual tailpipes with a free flow cat and muffler. it also has an AEM air intake setup. my problem is i feel the setup doesnt have the power that it should. the stock timing is supposed to be 0 deg. i had to time it at 4 deg. advanced so it would idle and run. it runs ok but if i try to get into the gas a little it lags badly. also gas mileage isnt near where it should be. i realise that a stock ecm(prom) will only go so far but this setup should have better throttle response. any suggestions on how i could improve the performance on this setup will be appreciated.

The problem might be that the free-flowing exhaust system may have reduced the back pressure to the engine to a point where cylinder scavanging is occuring?


Well if it wasn’t broke you should have left it alone!!! It has so much junk on it now it is confused!!

Do you have the right amount of warm air going into the air filter housing. The original one works best for that purpose. Without it, you lose a lot of power until the engine is fully warmed up, maybe even then. If you run the engine hard when it is cold, your plugs could have high speed glazing which reduces the spark.

The problem is that it’s still TBI, the GM TBI engines were always slugs. Swap in a multi-port fuel injection system and it’ll really wake it up.

the reason i replaced the original motor was it had 180k miles on it and it had no power at all. the only mistake i made was not knowing you really cant replace a motor with something other than stock in an OBD1 system unless you can program proms. notice how i didnt say buy new prom chips, they usually dont work right. i tried 4 different ones. you need to be able to change the values in the tables as you drive it.

actually it has a little bit of pep when its cold, probably because its still in open loop.

Maybe you didn’t time it properly. The fact that you had to time 4 degrees advanced to get it to run indicates a problem right there. Frankly it wasn’t uncommon to have a bad tip-in hesitation on this model year.

Did you follow this timing procedure?

Refer to the Vehicle Emissions Control Information label under the hood. Always follow the Vehicle Emissions Control Information label procedures first before using the following procedure.
Put the IC system in the bypass mode by disconnecting the “set timing” connector. This is a single wire sealed connector that has a tan with black stripe lead. This connector breaks out of the wiring harness below the heater case in the passenger compartment.
With the ignition switch “OFF,” connect the pickup lead of the timing light to the number one spark plug. Use a jumper lead between the wire and plug or an inductive type pickup. DO NOT pierce the wire or attempt to insert a wire between the boot and the wire. Connect the timing light power leads according to manufacturer’s instructions.
Start the engine, and aim the timing light at the timing mark. The line on the balancer or pulley will line up at the timing mark. The timing should be O?TDC . If a change is necessary, loosen the distributor hold-down clamp bolt at the base of the distributor. While observing the mark with the timing light, slightly rotate the distributor until the line indicates the correct timing. Tighten the hold-down bolt, and recheck the timing.
Turn “OFF” the engine and remove the timing light. Reconnect the number one spark plug wire, if removed.
Reconnect the “Set Timing” connector.

ive looked into that but it will end being a 2 or 3 thousand dollar gamble. what i really need is a programmable ecm like with the OBD2’s.

yes it was a concern that we had to go 4 deg. advanced. the procedure listed above is also in the service manuals and was followed to the letter. actually we tried to time it at a number of different settings and also tried higher octane gas but 4 deg. advanced and 87 octane was the timing mark that seemed to work best.

for two or three grand you could pickup a rebuilt, slightly warmed over LT1.

that i cant be sure of. the exhaust flows a lot freer than stock but i dont think its a backpressure issue. it feels more like an ecm that isnt properly programmed, and a fuel delivery system (TBI) that is lacking. those 2 components together i think might be the problem.

it has a modded exhaust with headers and dual tailpipes with a free flow cat and muffler…gas mileage isnt near where it should be.

Considering the custom exhaust system, I think your fuel economy expectations are unrealistic. A large diameter (or “free flow”) exhaust system usually leads to reduced fuel economy.