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Random hard start, mechanic cannot reproduce.

D

derherr65

NAXJA Forum User
Location
North Texas
I have a '97 jeep cherokee 4.0L I6. It occasionally will crank over and over without starting.

The battery and alternator are fine. Cranks at normal brisk speed.
Does not die or stall, though it does run slightly rougher than when I bought it.
It not putting out any codes.
I saw no signs of a fuel leak.
The pump is putting out 45psi average, though swinging wildly between 42 and 48.
My pressure gauge also has a outlet port for testing flow. Every time I check the flow it is proceeded by a burst of bubbles. Is there a way the fuel pump can suck air or cavitate? Air in the fuel would make for hard starting and rough running. Is the pump a rotor or diaphram type?
 
What does your fuel gague read when this happens? I think there was a recall on the 97 gague or pump (something to due with fuel). You might also want to consider other culprits besides fuel. CPS or failing starter?

It gets very expensive, while working on a Jeep, to wear blinders.----------make sure you are looking at all options and not just throwing money at what you think it might be.
 
Have not yet caught it acting up with the pressure gauge handy. I'll test fuel pressure and spark as soon as I can catch it acting up again.

Recalls & TSB:
Fuel sender may indicate more fuel than actual
Fuel pump grommet may deteriorate and leak fuel

CPS is certainly possible, but I don't think the starter is an issue. No dragging, no grinding, no solenoid click only, etc.

"make sure you are looking at all options and not just throwing money at what you think it might be."
I think that can be generally said of all makes. If you want really expensive to throw parts at, try my diesel Mercedes. You'll cry.
 
try running a can of Seafoam through the throttle body. You could have bad carbon buildup.
Have you pulled the plugs and checked their condition? and wires?

You should also turn the key and let the fuel system pressurize (about 15 seconds) before attempting starting. I recall reading something about using a BMW part in the fuel system to stop drain back out of the lines (Modified Forum about 1 week ago?).
 
A little more info...

All problems are in 'cranking' or 'warmup' mode as defined by the service manual. These are both open loop modes, meaning they do not use the oxygen sensors to correct the air fuel ratio, only the stock fuel tables. Since the majority of time is spent in closed loop modes 'warm idle' or 'cruise' without issue, I'm thinking this may be significant.

My current theory is that there is a intermittent vacuum leak. The O2 sensors can correct for this, except when the engine is not using them, in which case it runs lean and dies. Thoughts?
 
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Use the SEARCH function to look at threads about the check valve. On the 1997 to 2001 XJ's it is common for the fuel pressure check valve to fail. The symptoms are longer cranking times and rough running for 30 seconds to 2 minutes.
 
The nylon "sock" or screen that shields the pump's pickup could have disintegrated and clogged the pickup

Wierd gasoline and several types of injector cleaner can cause this sock to break down.

If the fuel pump is going to get changed, might as well drop the tank and make sure its clean inside, as well as change the fuel filter.
 
At first read, I thought the nylon "sock" or screen that shields the pump's pickup could have disintegrated and clogged the pickup.

The air bubbles could show there is a fuel leak. The only spot I know of that could admit air into the fuel lines without an obvious spill is the injectors themselves. Might pull them out and have them tested individually if it's worth the trouble.

But more than likely the bubbles routinely get in the schraeder valve during the pressure test, air gets in as the fuel spills out.

Could be a major case of vapor lock from funny gasoline, might burn this tank out and try a different brand.

-Vacuum leaks will raise the idle, as a rule.
 
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The nylon sock was changed about a year ago.

I finally got an interesting result on the fuel pressure gauges.(yes gauges, 2 of 'em, one electric, one mechanical)
Key in on position, pump whirrs for 2 seconds then stops. 0 PSI
Crank engine, little slow to start, but starts. 2-3PSI
Engine idles for 20 seconds. <5PSI
Engine idles and fuel pressure creeps up to 45 PSI.

That seems wrong to me. Shouldn't it hit 45 PSI very quickly and stay there? I didn't think it would run at <5 PSI.
 
what happens to the fuel pressure if you pull the vacuum line from the pressure regulator, (plug the inlet side of course), and go through a start sequence? Any difference?
 
If you have a bad check valve the fuel will drain back to the tank and any remaining gas in the fuel rail will now vapor lock. This would explain low pressure at the guage until it has started and run long enough to be able to clear the vapor.


Try turning the key to on until you hear the fuel pump prime, now shut the key off and turn it back on, back and forth 2 to 3 cycles. Next, try to start, it should be almost like normal.
 
Well, you can ignore the fuel pressure bit. The schrader valve adapter tip bent and wasn't opening the schrader valve, hence the funny gauge readings. I replaced the coolant temperature sensor as it was out of spec.
 
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