Again thanks for all the feed back.

We tested the fuel pressure last night and the pressure is not holding when you turn the key in the on position. it would read 36 psi and then as soon as you turn the key in the off position it would drop to 10 psi. My mechanic and i think, that we may have an injector or injectors that are sticking open or that are leaking, which would explain why the idle improves when we create a vacuum leak (more air for the motor). Since replacing the injectors require removing of the plenum, we are going to bite the bullet and go ahead w/ the top-end work. We already have 24# FMS injector, and now we'll start ordering the heads(trick flow) and cam (hot-cam). Hope the problem will go away after we've replace all the parts under the plenum.

thanks again. Let me know if you guys think we are off track.

joel

 

Sounds more like the fuel pressure regulator is the problem.It might be letting fuel pressure bypass it and return to the tank. Mine was doing the same thing and it was the fuel pressure regulator. just a thought hope its helpful. :cheers: :flag

 

The Helms manual has a procedure to distinguish between a leaky injector, a leaky regulator and another possibility - a leaking check valve on the pressure side of the fuel pump. All three will cause the line to lose pressure when the pump is turned off.

 

Here is a TSB that my be of some help!

Engine Calibration Change: to Improve Driveability and Eliminat
Number: Reissue 86-162

Section: 6E

Date: MAY, 1987
Subject: ENGINE CALIBRATION CHANGE: TO
IMPROVE DRIVEABILITY AND ELIMINATE FALSE "SERVICE ENGINE SOON" LIGHTS

Model and Year: 1986 CORVETTE
TO: ALL CHEVROLET DEALERS


Chevrolet Dealer Service Bulletin 86-162 is being reissued to provide a revised calibrator for use on the 1986 corvette with aluminum heads and manual transmission. All copies of Dealer Service Bulletin 86-162 (Section 6E) dated Oct., 1986 should be destroyed.

New service calibrators (Ref: called Mem-Cal in Shop Manual)
have been released for the 1986 Corvette with the 5.7L TPI
engine. These calibrators will address the following
customer comments:

1. "Rough Idle" on both hot and cold engine. The idle speed is raised to 625 plus or minus 25 RPM with automatic transmission and 750 plus or minus 50 with manual
transmission vehicles.

2. "Chuggle or Shudder" at low speeds - Torque converter clutch (TCC) engagement speed is raised to 41 MPH with automatic transmission vehicles.

3. "Surge" at road load - This improvement is for automatic transmission vehicles only.

4. "Engine Overspeed" in 1st gear - This improvement is for manual transmission vehicle only.

5. "Hesitation" on light acceleration with a hot engine.
6. "Service Engine Soon" lights - eliminates the following false codes:

A. Code 33 - Mass Airflow Sensor Voltage Low
B. Code 34 - Mass Airflow Sensor Voltage High
C. Code 43 - Electronic Spark Control (ESC) this correction is for cast iron head Corvettes only (VIN #119016 or earlier).



If normal engine diagnostics and repair do not correct the
above conditions, it will be necessary to replace the
calibrator in the ECM. Below is a listing of the new
calibrators and their usages.

I. On Iron-head Corvettes (VIN #119016 or earlier)

A. Manual Transmission use 1227943
B. Automatic Transmission with 2.59 (GM1) Axle use 1227942
with 3.07 (G44) Axle use 1227944



II. On Aluminum Head Corvettes (VIN #119016 or later) and all convertible models

A. Manual Transmission use 1228473

B. Automatic Transmission with 2.59 (GM1) Axle use 1228107
with 2.73 (GU2)/3.07 (G44) use 1228109

Use applicable Labor Time and Operation Numbers.

 

It could be a bad injector,fuel pump,or reg.
When I had the SAME problem on another MPFI/TPI car,the fuel pump was going out.It had a bad idle and went up and down.

A new fuel pump fixed the problem.Also be sure the relay is working as well.

You need to prime the fuel rail then pinch off the return line at the gas tank(remove gas lid door) and see if the pressure drops or hold.If the pressure holds better by the pinch test,then its a good bet the pump is on the way out.

If it still drops,look to a bad regulator or faulty injector(s)

With the car running,pull off the vacuum hose to the regulator and see if gas spits out or if it smells like gas.If any,it may be leaking.

Another test you can do,if everything else checks out fine,is to lift the fuel rail off the intake but dont disconnect the lines and see if any droplets form on the tip of the injecotrs.(car off)
Any droplets of gas is a leaking injector.

Hopefully its just the fuel pump.Its an easy job to change them out and takes less then an half hour or so and not as expensive as a TPI tear down for injector work.
:)

 

I talked to my mechanic, and he will check whether it is the regulator, injectors, or the fuel pump check valve, before installing the new components. I'm in the process of ordering the new heads, lifters, and cams. I plugged the numbers in desktop dyno, and his motor should be putting out near 444HP@6000 and 485FT/lb @4500 (at the flywheel) when using their HP manifold.(using full length headers gets 470HP :eek: ; but I can't find a CA smog-legal full length headers).

Again thank you all for your input. It really helped.

Joel

 

Mine did the same thing on me. The one way gate on the pump was bad.