rmytych

Hopefully I can get a few additional ideas on this issues. My 1991 Vette has 27,900 mile and is in very good shape... that’s I purchased it recently. It’s very hard to start, idle roughly and lack power when you jump on the go pedal. Here’s an overview of what’s been done and checked:

Standard tune up components and filters changed.

No error codes register on the scan tool

Fuel pressure is 40 to 41 PSI running. In the test procedures, turn the key on / engine off test; it jumps to 40 and falls off after a couple of seconds because there is no oil pressure signal.

I did check the oil pressure sending units and found one was leaking. I replaced BOTH since they were a beast to change (for me and my big paws) So the bad sending unit possibility has been eliminated.

HOWEVER... when you run the car, the fuel pressure is at 40 to 41 PSI. Turn the engine off and the fuel pressure IMMEDIATELY falls to zero. According to the book, it should hold fuel pressure. I suspected the regulator, but there is no tell-tale flooding or excessive fuel.

I can use a line clamp, pinch the fuel supply line (back at the pump under the fuel door assembly) and turn the car off. The fuel pressure not only remains IT SLOWLY GOES UP. (Never seen this) To me, that says the regulator is solid.

Any suggestions or tech advice for additional troubleshooting would be greatly appreciated. Need to get on top of this... the wife is calling this recent purchase a mistake. And we all know what comes after that... for sale signs.

whalepirot

The FP fallof is not due to the lack of oil pressure. Yes, the pump stops.

The big clues are " engine off and the fuel pressure IMMEDIATELY falls to zero", plus your pinching (probably) the return line to the tank. I suspect the FP regulator is bad (diaphragm), with steady PSI with that line pinched. If it fell, I'd suspect a leaky injector of rail.

Excess fuel (FP) is sent to the tank, 'decided' by a good regulator, based upon engine vacuum.

rmytych

Thanks for the info!

Yeah... The oil pressure comment was related to the hard start since the computer needs to see at least 4 lbs of oil pressure to keep the fuel pump turned on.

The line I pinched was the supply line. Doing that, the fuel pressure stayed would stay up... It even rose.

Should I repeat the test... Run engine and then PINCH THE RETURN LINE and watch fuel pressure. If it immediately falls off, it's the fuel pressure regulator?

I just want to make sure I'm doing the test correctly. Thanks!

Chad30021

Some of the symptoms that you mentioned seem indicative of a bad Opti Spark Distributor. At any point when the car is running do you have to put your foot on the brake and gas at the same time to keep it running or just on the gas if its a manual. When you are driving does it seem to misfire occasionally?

Possibly you could have some sort of a large vacuum leak in the intake system somewhere whether it be a vacuum line or leaking intake gasket or something of the sort. The best thing to do if that is suspect is to take some starting fluid and spray around all the intake gaskets, throttle body, vacuum lines, emissions lines and anywhere else after the throttle body where air can enter the intake. If the car idles diffferent when you spray the fluid then you know you have a leak somewhere.

On a side not my car does not hold pressure when you shut off the car but I have a C5 fuel filter and pump mod so I do not use the factory diagphram. however that does not affect the car running in any way. Hope this helps

rmytych

Thanks for the additional ideas. As far as being difficult to keep running, the answere is no. The car hunts a little at idle, but does not try to stall. I've also looked it over for obvious cracked vacuum hoses. The starter fluid along the manifold is worth a try. Gotta find the problem.

I've seen similar immediate fuel pressure fall offs with GM SFI spider systems that are located inside the intake. That's typically the FPR. Those usually come with extreme flooding and excessive raw fuel. My Vette has neither... Which is why I'm scratching my head. I'll keep testing and throwing parts at it.

Cliff Harris

The oil pressure switch is not connected to the ECM and it is a backup that runs the fuel pump if the fuel pump relay is bad. The ECM needs to see DRPs (Distributor Reference Pulses) from the ignition module inside the distributor before it will turn on the fuel pump. DRPs are generated any time the engine is rotating, either cranking or running. The ECM uses the DRPs to synchronize the injector pulses to the rotation of the engine, so the fact that the engine runs shows that they are OK.

Immediate falloff of fuel pressure could be a leaky injector, bad fuel pressure regulator diaphragm or a bad check valve inside the fuel pump. The fact that pinching off the supply line causes the fuel pressure to stay up says that the check valve is bad. Usually this is not a problem as long as you have adequate fuel pressure when the engine is running.

GKK

The 91 Vette has the Multec fuel injectors that are not compatible with todays ethanol fuels which eventually dissolve the injector coil insulation and short out the injector so, I would replace those first with the standard rebuilt 22lb. Bosch II or Bosch III's from supporting member Jon Banner here: http://fuelinjectorconnection.com/sh...php?groupid=72

Next, I would clean/replace the IAC (Idle Air Control) valve and passageway located on the throttle body which controls the idle. Also, check the TPS (Throttle Position Sensor) voltage, which should be .54 volts with the throttle held all the way open with the engine off.

Assuming the engine timing and regular maintenance items are working correctly, this should get your 91 Vette operating a lot better!...






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rmytych

Thanks!!

VikingTrad3r

Hey, i bought an 85 and it ran quite well. But when i did the fuel pressure test, it would be great while running (39) but it would bleed off almost immediately.

i also had a slight fuel smell every once in a while that would waft into the air.

turns out a PO had jacked up the rear driverside and i can see the impression in the jack that a hole in the gas line. i think its the return line.

when i spliced in new line, my fuel pressure no longer bled off after turning the car off.

i get it may be the check valve at the pump but maybe have a look at the lines and see if they are punctured?

good luck meng.

pcolt94

I was thinking injectors also.

I believe the opti was in the years 92 to 96. The 91 does not have an opti.

aklim

I have a 91 and it has NO opti.

aklim

I can see taking out the entire TB and the IAC housing, cleaning them and the passages throughly but there is no way to adjust the TPS. That comes with holes for the bolts and not slots. IIRC, it takes the startup voltage as base.

aklim

If he has fuel pressure drop off, something is up. How do you pinch the line going up to the rail and have higher pressure when you are clamping off supply to the measuring point, the schraeder valve?

Also, since the injectors are multec and probably junk and not worth anything, why not replace them and the fuel pressure regulator at the same time? All it is would be to change the diaphragm and that isn't horribly expensive.

GKK

Please, note that I replied, "Check" the TPS voltage.




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rmytych

All Great ideas. Thanks!

Budget right now is not going to allow new injectors. Ordered FPR diaphragm kit and plenum gaskets kit. Will change the FPR and clean the throttle body good and see what I get.

Gonna' try to do that over the weekend and will let y'all know the outcome. If the problem still exists, I guess a new set of injectors are in order... But that will probably have to wait til after the holidays.

rmytych

Well... After doing a little more research it would probably be best to put the new FPR on the shelf until I can get a new set of injectors and replace the injectors and FPR at the same time. I had no idea that the Multec injectors were so problematic. I'd hate to break it down to install the FPR only to have to do it a second time for injectors because the starting, idling and running issues didn't clear up.

aklim

You can use reman injectors which are under 200,IIRC.

rmytych

Yeah... I was checking out the videos from fuelinjectorconnection.com and his pricing. He has a lot of YouTube videos on the topic and is a forum member, too.