awork

I have a 97 cherokee sport with the 4.0. Here lately i have had issues when it is cold and I start driving. It will begin cutting out on the first acceleration. Normally it won't let me go past 2000 rpms and after about 10 seconds or so it will drive fine with no issues. I checked the fuel pressure and started by just turning the key to the on position first, it went up to around 25 psi. I shut it off and within a minute or so it had dropped to nearly zero.
I cleared the gauge then started the jeep and let it idle. At an idle it was around 43-45 psi. I gave it a little gas and it would not change (i assumed it should go up). After shutting it off the pressure steadily dropped to under 30 in less than a minute. From what i have read 49+/- 5 is normal but i don't believe it is suppose to drop that quickly.

Any advice on if this could be the issue and would the fuel pump need replaced?

Thanks

s346k

my fuel pressure does the exact same thing, but the engine functions perfectly fine regardless of temperature. the reason your fuel pressure is dropping after cutting power to the pump is because the check valve in your fuel pump assembly is bad. it's a common problem, remedied by cycling the key on and off a couple times before starting the engine. aka the poor man's prime. the pump itself is fine, the cold engine stumble is not related. have you done any work to it recently?

awork

Thanks for the reply. I actually tested it again and left the hose on for like 20 minutes and it only dropped about 10 psi. I have actually done a lot of work lately. I had several issues with it. I have replaced plug, wires and distributor cap. Had a somewhat similar problem but 20 times worse before. It started slow but began barely running at all. I cleaned the throttle body and IAC. It continued to get worse so i tried replacing the TPS and did not fix it. I took it in and got it checked out and they said it was losing connection at the crankshaft position sensor so i replaced that and the 02 sensor since it looked pretty bad. That fixed it for about 5 minutes then went back to normal. I took it in again and another place said it was the MAP sensor. I replaced that and it has been running great for the last 2 weeks. The problem i am having know only happens when you first take off, once it warms up it is fine. But it will just stall out like its not getting gas maybe then after about 10 seconds or less its runs perfect. I have the codes ran and no codes besides the Neutral Safety switch are showing up (it was bypassed by previous owner)

s346k

you could always hook that gauge up to the engine and drive it around to see what the fuel pressure is doing in relation to driving. I seriously doubt your problem is pump related, but it never hurts to be 100% sure. my pressure dropped from 48 lbs to 8 lbs within 3 minutes of turning the key. it has been like this for 30k+ miles and 1.5 years.

CCKen

Generally a defective upstream O2S will cause stumble out of the gate but behave normally when the engine heats up a bit.

Check the O2S heaters fuse in the PDC. It's a 15A mini fuse in the top row of fuses (as you're looking at the PDC from the radiator), 2nd one from the left. To be sure, check all the 15A mini fuses.

awork

Ya after checking the fuel pressure I was thinking it may be something else. Which is the upstream 02 sensor? I have replaced the one closest to the engine, it was pretty dirty but never did the other one.

awork

I also checked all of the fuses under the engine just the other day and none of them were blown.

s346k

upstream is the one nearest the engine. the o2 in the catalytic converter is just a tattletale, it does absolutely 0 with regard to the engine.

awork

I am not sure if it was coincidence or not but this morning I turned the key 3 times before starting it and it did not cut out. I am going to try the same thing tonight and see if it does it again.

Roler

Besides the checkvalve, you can also lose the fuelpressure due to leaking injectors. The stumble in that case is causes by the engine clearing the excess fuel.
One way to differentiate between those two causes is starting the engine (fuel pressure gauge connected) and clamp off the fuel line immediately after you turn the engine off, do this as close to the injectors as possible. If your fuel pressure holds up, or only drops very slowly compared to your first reading, the problem is between the point of 'clamping' and the fuel pump. If it drops just as fast clamped, the cause should be sought between clamp and injector.
Never had to try this myself (so can't point out the exact point where should be clammped) but the above is the theory behind this diagnostic procedure.

FYI, the priming of the fuel pump as stated (turning key on and off a few times, before starting) goes by the name " poor man's prime" and can be done safely. It's not damaging anything. Been doing it for 1-2 years now, because I can't be #$%^ to change out my fuelpump (where the checkvalve lives).

awork

So I am beginning to wonder if it's a fuel pump issue. Half of the time priming it makes it not have the issue but half of the time it still does it. Drove it today without priming and no issues. Stopped at a gas station and came back out and it cut out for a few seconds again. Any ideas?

s346k

have you pulled and inspected the injectors?

CCKen

My money is still on a defective upstream O2S.

Oxygen sensors should be changed in pairs. Both should be NTK sensors in your case.

Never mix a new downstream with an old upstream O2S.

Never mix brands, like NTK with Bosch.

Before changing O2 Sensors always examine all visible O2S wiring for chafing.

Alway check the O2S heaters fuse in the PDC.

The O2S heaters are grounded at the ground studs at the ignition coil.

BTW: The downstream O2S does interact with the upstream O2S to set a goal voltage. A wonky downstream will cause poor fuel economy.

s346k

I contest these statements, as I have done both of them with 0 ill effects. when I first got the jeep I replaced just the upstream o2, no negative effects - even 30k+ miles later. I also recently disconnected the downstream o2, and alas...no negative effects. the jeep consistently gets 22+ mpg and runs as well as it possibly can.

awork

So if one of the o2 sensors are defective could I try un hooking One At A Time And See If It Makes A Difference