help with fuel pump diagnosis

this is a 91 cherokee with the ho 4.0 engine.during the last very cold blast here it would not start after i went to the store.after a bit i got it going and home(just a mile or 2)the next day it still wouldnt start.i suspect the fuel pump.i disconnected the fuel line forward of the large filter mounted on the framerail and ran it down into a bucket.then i turned the key to see what kind of flow i had.(i know it only pumps for a few seconds when you first turn the key)it did seem to flow but hardly anything from what i thought it would.it came out but no more than really if i had gotten a siphon going manually.then i broke the line before the filter and ran it into the bucket.i thought this way i could eliminate a clogged filter.the flow was the same.

im an old school carb guy and im not sure what im supposed to see when this high pressure pump is pumping.

shouldnt it have blown like mad with little volume but lots of pressure?

i bought a complete sending unit with a new pump but i just want to make sure my diagnosis is correct.

any insight is greatly appreciated

thanks

Your logic is correct and it likely is the pump to blame, but if you would like to confirm, get a hold of a fuel pressure gauge (can be rented at many parts stores) and hook it up to the schrader valve on the fuel rail. If memory serves me correctly, I think you should be around 31 psi; someone please correct me if I'm wrong on this spec. But the engine does have to have sufficient built up fuel pressure in order to start.

May as well get that filter out of there as long as you're at it.....good preventive maintenance unless it's been done recently.

thanks!

Correct, it must have a built up pressure to start. I just confirmed my pump is bad from your advice in my other post birchlakeXJ. Thanks again!

ok so my son and i changed out the whole pump assembly tonite outside in the freezing cold.new hoses and filter too.cranked it over a bit several times and it still wouldnt even try to start.i pushed in on the schraeder valve on the fuel rail and it blew out like mad.i dont have a gage on it but im pretty sure there is plenty of pressure.i pulled the coil wire and ive got plenty of spark there then i pulled a plug wire at the cap and again the spark seems good.we cranked again for about another 7 min or so untill the batt went dead but it never tried to run.im at a total loss now.
the chk eng lite has not come on at all either.
the only thing i may have done wrong was that i left the gas cap off when i drained the tank down.it was off the whole time i was trying to get it to start

any ideas?

ive got to get this moved tomorrow nite somehow.

thanks

mark

Have you checked your spark plugs? They could be gas fouled. Sometimes they will clear out if you crank it with the gas pedal floored. I would just pull one out though and take a look. You don't want to run your battery down again.

The battery MUST have a good charge and voltage while starting. The electrics poop out before the starter stops turning over.

--Fully charge the battery or find another fully charged and try again.--

Good Luck,
Orange

thanks ill charge it tomorrow and give it another try tomorrow nite.if it sits for 24 hrs do you think i still need to pull a plug or should the be dry by then if they are wet with gas?

if i am getting spark can i rule out the crank position sensor?

also should i even check to see if maybe there is a blown fuse for something?

Carb or FI should be the same, just PSI different. Two Jeeps same motors and fuel mileage- say 15 mpg at 60 mph would burn 4 gallons per hour. So SWAG say double+ for full throttle = say 10 gph pump output...so 1 gal in 6 minutes, 1/2 g-3 min, so on.. Just a guess!

IF the 'no start' troubleshooting bogs down, later might try a timed test fuel flow. If it has spark/fuel seems like it should at least try to start even ifs not exactly the right mix.

Might start a new thread about no start since its changed to that.

Back to basics: air,fuel,spark. narrow it down till fixed.

Good Luck,
Orange

markg said:

if i am getting spark can i rule out the crank position sensor?

Click to expand...

Yes. But you should verify that you have a strong, blue, snapping spark. Orange or yellow spark = weak spark which may not be strong enough to start the engine. This is really important. If you have weak spark, test your ignition coil.

Leaving the gas cap off should not have created your no-start.

I agree that pulling a plug or two for gas fouling (flooded) is a good idea. If they are wet, gotta get em out and either dry or replace them.

And be sure battery is fully charged before trying again.

I want to say 49PSI at the rail, but I am not sure.

Not on a 91. 31 w/fpr connected, 39 w/o. The 96+s are 49psi. If you're gonna make ish up, make sure it's beautiful ish. . .

im about ready to throw in the towel now.
me and my son have done everything ourselves as far as work goes since he has had it for over 6 yrs now. we have done everything from puting 1 ton axles under it to general maintenance and even did the rear main seal twice.we have always worked thru every problem or condition flawlessly.
tonite i bought a can of starting fluid and sprayed it in the intake tube while he cranked it.nothing at all!it didnt even kick once.it just cranked and cranked.
then i pulled a plug and noticed that it was very worn and appeared to be a little wet.i put the wire back on the plug held it to a ground while he cranked and i did get spark but not as strong as i thought it could be so off the autozone we went.we bought new autolite plugs and a new coil.
then the moment of truth.
he cranked and cranked and still nothing.
i pulled one of the new plugs put the wire back on grounded it and the spark was really good now.
so far i have replaced the fuel pump,hoses,filter,coil and plugs.
i am at my wit end right now.
while cranking i was watching the tach and it did not jump or move at all.there are still no engine codes coming up either.

any ideas?
i asked autozone if they had a cam sensor and they claim that there is none on that year(91)but they do have a crank sensor.
should i try the crank sensor or do you think this will be another waste of time?

oh and i forgot to mention the we were cranking so long that towards the end i must have burned out the neutral safety switch or something because it now will only crank over in neutral and does nothing in park.

If you have spark and fuel I would doubt the crank sensor is bad...the cam sensor? I dunno.

As far as AZone, I asked them for a CPS (CKS) they told me their computer said it wasn't needed. I, disgusted, went to NAPA.

Just a few cheap guesses--

-The relays in the engine compartment might be in the picture. might try swapping the same type around.
Test.

-Try the unplug-plug few times on the CPS.
Retest.

-Good Strong light and recheck areas worked for wires pulled loose, etc.

-Try starting while holding the the throttle open about 1/4 till she catches.

Past that -change CPS, (save old one for spare)

Good Luck,
Orange

If you have confirmed good spark, the crank sensor is almost assuredly not the problem.

The cam sensor (yes, you have one no matter what the guy at the parts store says; it's in your distributor) is in play here. Not as common as a crank sensor for a no-start but it can happen. Be sure that the distributor cap and rotor are in good condition; replace if any doubt. Failure to replace secondary ignition components can sometimes result in the chasing of ones tail.

Cam sensor can be tested with a manual and a meter.

I'd really recommend that you get a meter and start testing stuff before replacing it. It's the only efficient way of getting under this.

gradon said:

Not on a 91. 31 w/fpr connected, 39 w/o. The 96+s are 49psi. If you're gonna make ish up, make sure it's beautiful ish. . .

Click to expand...

yeah, sorry about that - that's why I put the "not sure this is right" disclaimer on it. I knew I saw 49psi mentioned somewhere on this forum but I was not sure which years it applied to.