Chev V8 mileage dropped 15+ to 10 in OT OBD1 TBI

Alright here are the details

I installed a stock 1993 TBI 350/7004r in my 1970 Chev C-10, complete with stock OBD1 computer, stock 1990 Suburban fuel tank, pump and pickup, painless harness, 700r4, 3.73 open diff. Motor has 140+k on it and runs good, starts great, no idle problems, no stalling, no stumbles, motor seems a little tired but it has 140k on it and was only 185hp new. This series of motor/computer has only 1 before CAT O2 sensor.

For years I was getting 15+ mpg in town with limited highway driving, about 6 months ago the mileage started to get progressively worse, I'm the only one who drives it, no change in driving habits. I've inspected the fuel lines, no leaks, I've pulled the air cleaner and watched the spray pattern from the 2 injectors not a perfect mist no running rich smell, no leaks around TBI. I had a friend throw it on his scope, he said the O2 sensor was responding very slow and I had a small exhaust leak at the manifold, I changed the O2 sensor and replaced the exhaust gaskets. I pulled and replaced the plugs, even tan color no build up. No improvement.

As of last week I'm getting 10.3 mpg, I don't expect much from this daily driver but I might as well swap in a 427 fire breathing monster for 10 mpg.

I hate to send it to a mechanic for a fishing expedition but if thats all I've got left?

I need another year of service out of her before I can think about replacing the motor.

Thanks guys

 

did you reset the computer...? and check the egr

 

Please take your OT discussion to a proper forum. Thanks.

 

check the MAP sensor...Look for leaks in the hose/tubing connected to it. What kind of manifold vacuum do you have at idle??

 

I left the computer uplugged overnight, no power. I haven't looked at the EGR, I did take off the smog pump stuff when I installed it years ago. The EGR rattler on the valve cover should be cheap enough to replace.

I did spray a ton of starting fluid all around the intake and base of the TBI to check for vacuum leaks, no jump in RPM so I guessed no leaks.

Thanks

 

happen to me, fuel pump is low on pressure and not spaying currect.

 

have you checked the distributor? cap & rotor arcing will reduce mileage.

 

The EGR valve is not on the valve cover. It is round, bolted to the intake manifold and has a vacuum line to it. And, it will cause your problem.

 

I'll throw my vaccum gauge on it this weekend

Ok, I misunderstood, I believe I had to replace that years ago and it was on the very back of the manifold, I'll replace that again.

I guess if anyone else has some thoughts, PM me.

Thanks guys, great information

 

I dropped 4mpg by just using ethanol fuel. And it stinks too.