C4 Question

Wathen1955 · 07-15-2013, 9:35pm

Hi gents,
I sent a question to Friz, but don't know how often he visits here. Anyway...

I have a 96 that is stock. Today was the second time within a month where it took me half a minute or so before she started in the morning before going to work. Previously, it would fire right up within 1 second. The car is driven once a week - 30 miles each way to work. She has 104k miles on the clock.

My guess would be something to do with the injectors or losing fuel pressure when the car is shut off over time - a leaking injector?. Once she starts, the she runs just fine. Even leaving work this afternoon it fired right up like always. So just guessing again, seems like there is no fuel in the rail when I go to start it.

I'm not an expert at fuel injection so I'm guessing in my diagnostic attempt. I don't have a way to check the fuel pressure (don't have a gauge). I guess getting one and seeing if the pressure drops when shut off would be a clue? The other times where it sat for a week (in between this issue), she fired right up so I'm lost - intermittent injector?

Aerovette · 07-15-2013, 11:19pm

After it sits a while ( the normal time that you see the problem) go out and press the Schrader (sp?) valve on the rail and see if there is pressure. If none, then your suspicion is correct. I think also that the pump primes when the key is first inserted so you may want to do it before you put the key in. Ihave not had my C4 for a while and have forgotten a LOT about it. Loved that car though.

mike100 · 07-16-2013, 11:25am

You will have to invest about $40 for a fuel pressure gauge or ask around to borrow one. Have a helper start the car in the morning while observing the gauge. You ought to have around 40 psi (I don't recall exactly what the LT1 fuel pressure regulator is set for).

You should have pressure pretty much right away. Usually if regulators go out, then you have too much pressure causing the closest injector to be forced open- sounds more like the fuel pump and/or filter causing low flow. luckily the fuel pump is one of the easy jobs on a C4- probably easier than the fuel filter, actually.

Wathen1955 · 07-16-2013, 3:15pm

Thanks Mike. So the regulator is part of the fuel pump? Yesterday when driving home, I came to an open area where I could punch it, and it did not hesitate or sputter at all. Would this still indicate the fuel pump causing low flow?

mike100 · 07-16-2013, 5:18pm

The regulator for the 90's cars in on the fuel rail with a vacuum nipple on it. It raises the fuel pressure by that diaphragm when the manifold vacuum drops on acceleration. When they fail, there will be gasoline on the wrong side of the rubber diaphragm (check for gas by removing vacuum hose). This makes it run horribly rich and is not your problem.

Maybe the fuel filter inlet sock is fuel of crud...usually you just replace both at the same time. As easy as the pump is on the C4, I would consider changing it as a preventative maintenance item.

Frizlefrak · 07-17-2013, 5:53pm

I've been busy last few days, hadn't been on here.

You can borrow a gauge from Autozone for free. Hook it up, turn key on but don't crank. Give me that reading and we will go from there. I'm suspecting a dead fuel pump relay is causing the long cranking times. Fuel pressure test will confirm.

Wathen1955 · 07-17-2013, 8:37pm

Thanks Dan. I won't be able to check it until this weekend and I'll let you know.

Wathen1955 · 07-20-2013, 11:23am

I have only 18 psi. When I turn the key off after a minute, it drops about 2 psi and holding. The car has been sitting since Monday. When I first connected the gauge, there was no pressure at the rail. Does the pressure slowly bleed out over time?

mike100 · 07-20-2013, 11:36am

its less than 1/2 of what it should be- lucky it still runs and you haven't had to pay for a tow. It is possible it will slowly increase once the engine is running, but I can see why you would have to crank it over so long at 18 psi.

The fuel pressure may bleed down over a long time, but in general, it should be able to hold some pressure after you turn it off.

Frizlefrak · 07-20-2013, 12:23pm

It's a wonder it runs at all.

Pinch off the return line with vice grips, repeat the key on test several times. Tell me if it changes.

Wathen1955 · 07-20-2013, 2:17pm

That's the return line from the filler in the rear correct? Maybe I'll just go out and get a new fuel pump. I noticed this morning when I turned the key to the on position, I did not hear the fuel pump energize.

Any recommendations where to buy? I usually get parts from NAPA.

Frizlefrak · 07-20-2013, 4:23pm

Quote:

Originally Posted by Wathen1955

That's the return line from the filler in the rear correct? Maybe I'll just go out and get a new fuel pump. I noticed this morning when I turned the key to the on position, I did not hear the fuel pump energize.

Any recommendations where to buy? I usually get parts from NAPA.

And when you put the new pump in and it still does the same thing? You'll be $100 poorer with a car that still isn't fixed.

Troubleshoot....don't throw parts and money at it. You can pinch off the return line by the tank and do the key on test. If it suddenly starts building 40 psi at key on, the regulator is bad. If it doesn't, it could be a bad fuel pump, split pulsator, leaking injector(s), or clog on the pressure side. Time to start running more tests.

If you're not hearing the pump come on at key on, the fuel pump relay is likely bad, and the car is having to rely on the oil pressure switch to energize the circuit. This would also explain long cranking times.

Run tests until something fails a test.

Wathen1955 · 07-21-2013, 11:29am

OK. I pinched off the return line and did the test again. No change in pressure. I did the key on/key off about 10 times. Still at ~18 psi. I then started the car and the pressure shoots up to 38 psi.

mike100 · 07-21-2013, 11:56am

Is the fuel power system the same as the older TPI cars- relay on ignition power, with a parallel feed anytime the engine has oil pressure?

Maybe you could check the relay. God knows where it is in the later cars though- hope it isn't under the dash.

Wathen1955 · 08-09-2013, 7:39pm

UPDATE:
Turned out to be the fuel pump. The new pump shows almost 50 psi when ignition is turned to on. When started, it drops down to 46-47 psi.

The strainer was black in color and split down the middle so it was of little value in doing what is was suppose to do.