Martin082514

Hello I recently bought a 1987 Corvette fixed all interior and now trying to start the engine but when it's in parked with the engine running the rpm stays at 6-7 RPM and when I step on the acceleration while on park and let it go it drops down to 3 RPM and oil pressure drops down to 32psi the lowest then goes back up to 40psi I just changed the idle and tps please help what can be the reasons my RPM drop and won't stay steady it stays normal 6-7 RPM but while parked and me accelerating it goes up but then letting it go the RPM goes down to 3-4 RPM as if it wants turn off on me what can be the reasons and how to fix it? Please help

billschroeder5842

You need to check your fuel pressure. If your fuel pressure fluctuates, you could be experiencing this rpm

Could be your FPR, Clogged filter or weak(ening) pump.

My guess is that your FPR is on the way out.

Martin082514

What is the FPR sorry not much of a mechanic but doing everything on my own with help of those who know more about corvettes than me corvette is my dream car but really don't know much of this so what is FPR please?

Martin082514

Sorry it is the Fuel Pressure Regulator right? And that would be the only thing that would cause my corvette to do what it is doing?

aklim

How did you "just changed the idle and TPS"? I know you can change the TPS but without tuning, you can't change the idle unless you screw it up first.

When you accelerate and then let the pedal go, the rpm drops and thus fuel need drops off too which might explain why it is at 32 psi. If you really want to test it, put the pressure gauge on, turn it to "run" and watch what the pressure is and then turn it off and see how long it holds. After that, do a Wide Open Throttle run with the gauge taped to the glass and see what the pump does under load.

You started a fuel pressure check. Congrats. Most don't. So now we need to take it to the end and see if the pressure holds after shutdown and whether it performs under load.

Next we need to see if your spark is good. Is timing on? You didn't mention. Are your plugs good? Cap and rotor good? Coil good and giving a blue flame? Plug wires leaking?

If you can get through all that, get a scanner, NOT a code reader. A Scanner and see what the IAC counts are and whether the Coolant Temp is right based on the ECM readings and an infrared thermometer scan of the CTS.

kael

Martin, I know how you feel, I switched to mechanic myself for my lovely Lady.

First thing you'll need to check is the fuel pressure. You'll need a fuel pressure gauge, it will have a hose that you connect to the right side of the engine. Search for this, you'll find some pics or youtube to clarify.

If you plan to keep doing as much as you can yourself, get a Field Service Manual (FSM) for your '87. You can find digital versions for $30-40. Keep reading here on CF and search a lot, let me know if you need a Google way to search. A Hayes manual for '84-'96 Corvettes is good, more pics and some little tidbits. Don't bother getting a Clinton, not good.

I got my own '87 2 years ago and since have moved from replacing a headlight to actually removing and installing heads over two years! First time ever for me. There is some cussing involved in doing some work, but much more cussing after reviewing what some paid, as-fast-as-possible, mechanic did to our dreams.

Good luck going forward, hope you get to feel the Drive soon!

billschroeder5842

No, there are several things that could cause your condition. The FPR is one of the first (in a series) of things to check right after verifying your overall fuel pressure and how it responds/holds with rpms go up and down and after you turn the engine off.

gerardvg

Hi
The usual culprits are vacuum leaks, exhaust gas recirculation valve sticking open. I would start by clearing the stored codes, disconnect battery for a few seconds and reconnect.

Does the check engine light come on?
Your car will show a check engine light if anything is out of spec, you can put a paper clip or wire in the two top right terminals of the connector under the dash.
Turn ignition on and write down the amount of flashes, ie 4 flashes pause 3 flashes that will be code 43 for example.
Write them on a piece of paper and google the codes, then you can identify what may be a problem.

Start it and see how it runs, it should start run at 1200 rpm for a few moments then drop to 600 rpm settling around 580rpm hot.

If the idle screw has been adjusted you will need to do a base idle setup procedure, google c4 corvette base idle setup.

Your oil pressure is not a problem it will go up and down with engine rpm, your fuel pressure should be 40 psi.

The usual new spark plugs ignition leads and check timing are a good way to start off, is usually a small issue these cars are pretty reliable.
Old cars need new parts from time to time but your car will tell you what is wrong via the stored trouble codes (whenever the check engine light comes on)


Good luck

aklim

I wouldn't trust the paperclip. I don't really think the 30 year old tech is worth much when it comes to this sort of thing. I believe it only gives a code when the value the ECM sees is NOT PLAUSIBLE. IOW, if your temp sensor is out and reports -20 degree, it is plausible so it won't throw the light. I think monitoring the sensor via a scanner to the PCM so you can see the datastream is going to work much better.

Assuming the tach is right, yes. Quite often, I have seen that it doesn't jive with the ECM or my timing light with a tach.

Another thing that will ASSUME that the timing is right and tach reading is right and there are no air leaks which affect the idle.

I'd pull the hose to check fuel pressure. This will help ensure that the diaphragm isn't leaking and it simulates a WOT condition to see what the pump would do. After which I would reconnect it and go for a WOT run to ensure that the pump can keep up under load, go home and shut down and see that it holds the pressure for a while before you declare that part done.

I would qualify it with "Only if the value is FUBAR".