Ultimate G

I’m having a strange problem getting fuel into the engine. I can’t get any fuel pressure if the engine has been run briefly anytime within the previous 6-8 hours. However, it will work perfectly if it has not yet been started today, or if it has started, and then been allowed to fully warm up.

I’ve searched all over, but not yet seen anything that would be so intermittent like this. It does crank just fine, and appears to have good spark. The fuel pump makes its normal sound when I turn on the key. I can hear fuel being delivered to the engine when the pump runs. I got a faint whiff of unburned fuel under the hood after cranking. Tank is half full. Fuel filter is six months old. I’m testing fuel pressure by pressing on the Schrader valve with a screwdriver. When things are working, I get a brief little squirt out of the valve, lasts for about 1 second. If it’s not working, only a drop will seep out.

So, to recap:
It starts & runs great if the jeep has sat for a day.
It starts & runs great if the engine is hot.
It won’t re-start if the engine has been run briefly, turned off, then started again.

If it does start it will run nicely - when Idling and pulling down the driveway – (But I haven’t highway tested it since the problem began).
BTW, if it matters - The day before this problem appeared, I did an oil change and added 2oz of MMO to the gas tank.


Any suggestions?

1994 XJ, 4.0, AW4 All-stock daily driver. 126k, in otherwise great condition.

SeriousOffroad

Have you actually used a fuel pressure gauge on it?

Ultimate G

I've got one on order at Napa. Unfortunately, It'll be a while before it gets here.

The only fuel pressure test I've done is to use my finger to open the Schraeder valve on the fuel rail and see what comes out.

SeriousOffroad

Hmmmmm. Sounds almost like it's flooding the cylinders. Next time you get a no start condition, pull a couple spark plugs and see if they're wet.

BuckB91XJ

I guarantee you that your fuel pump is bad. My 87 XJ got to where it wouldn't even start but, you could disconnect the fuel line at the fuel rail and turn the key on and it would shoot gas clear across the garage. You have got to have 31 PSI +or- 1 PSI, when you put a gauge on the schrader valve, or the fuel pump is bad...PERIOD! Trust me on this one. I've played this game too many times.

Ultimate G

Frank, you da mang.

Fuel pressure is good, even when it's not starting.

So, I pulled a plug and it seemed dry enough. After a few unsuccessful attempts at starting the normal way, I held the throttle to the floor and cranked it, and it fired right up! (I've heard starting with wot will tell the ECU that it's flooded, and the injectors won't fire)

Now, how do I fix this?

Thanks
G



Edit: Come to think of it, I've been down at sea level elevation for the last two week, and just drove it home to 6500' the night before this problem arose. Think it could be the computer hasn't caught up yet?

.

SeriousOffroad

How were the Engine RPM's when it fired up with the throttle open?

winston427

It may not be what is causing your issues, but may be something to check down the line. Kind of a last ditch effort when all else has been ruled out before you replace the pump. When I bought my 89 it would take way to long to start but would idle fine when it did start. If the throttle was applied slowly it would build RPMs with no problem but would die instantly with a rapid throttle pressure. It finally stopped running all together and had zero fuel pressure. After going through the check list of possible causes and ruling each one out I was condemned to pull the pump out of the tank. The previous owner had installed a new pump but neglected to secure the pickup hose and it had come off inside the tank. 3 hours of aggravation only to find that it was a simple issue

Ultimate G

It seemed totally normal. The engine fired right up on the first revolution, and I instantly took my foot off the throttle when it caught.
It immediately settled down to a nice typical, smooth idle at 800. It probably peaked at about 1500 or so when I cranked it.

SeriousOffroad

Hmmmm, very weird. How about a very thorough cleaning of the throttlebody with particular attention to the IAC.

Got a spare TPS laying around somewhere?