Help With a 2000 Cherokee 4.0: Horrible Mileage and Labored Starting...

[GJStringert]

Hello all,

I'm posting this for my girlfriend. I usually hang out in the fullsize section, but since EXPO and cumminsforum are the only two forums I'm a member on I wanted to hop over to the Jeep section and see if you guys had some handy tips!

Here goes: it's a 2000 Jeep Cherokee, 4.0 auto with around 104K on the odo. This is my girlfriend's daily driver, and it's never been offroad (not yet at least!), and we DESPERATELY need to get it running right, poor girl doesn't have the funds for 6 mpg...

First big problem: it starts REALLY hard, be it hot, or cold, having just been run, or not running for a week. Now I have a little experience with the 4.0 motors, and I've heard about the fuel pump anti-flowback valve (anti-siphon? I can't really remember the technical name for it) which holds fuel upstream from the tank in the lines, so to speak. We replaced the fuel pump last January, never solved the problem. Once it's running, it runs absolutely fine. On occasion, it may idle just a little rough, but honestly you'd need to be really looking for it to notice, it seems to run and drive absolutely fine. I'm stumped here...

The next big problem is recently, like the last 4 tanks of gas, she's been getting atrocious fuel mileage (see below picture).



It's 80% city, 20% highway, roughly, and she's a pretty conservative driver. Things I have tried: replaced O2 sensors 2 weeks ago (they were filthy, I was convinced that would solve the problem but it hasn't), cleaned air filter, checked the spark plugs (they're very clean, gap's good, etc), checked every vacuum line I could find, and I was told by a friend that only the grand Cherokees of this generation had serviceable fuel filters, so I haven't really dug any deeper on that (point me in the right direction maybe??).

So I'll take ANY advice I can get, I really appreciate it folks!

Graham

 

[SDDiver5]

You check the injectors?

 

[GJStringert]

Thanks for the replay, and yes I forgot to mention that. The injectors were replaced a few years ago before she started driving it, I'm guessing around 15 k ago.

 

[SDDiver5]

Strange... I know a little about the 4.0 motors but I dont have any more advice other than take it to a mechanic and ask them to run some tests. Good luck.

 

[duckhunter71]

The starting issue is likely the check valve (it does as you described - keeps fuel in the line to the engine after the ignition is shut off and the fuel pressure drops). My 2001 had this issue and to remedy it I would cycle the key a couple times to prime the fuel pump before starting and then I would turn the key all the way. It would usually spit and sputter a bit after sitting for a long period of time, but I could usually get it to even out with a little bit of gas pedal. It would intermittently make the CEL light come on, but I just learned to ignore it.

As far as the fuel economy goes, I don't have much to advise. My 01 was lifted 4.5" with 31" ******** Cepek FC-IIs on it and it got 15-18 consistently. Good luck with your troubleshooting!

 

[redveloce]

It could be a plugged cat. How is the power?

 

[docwatson]

It could be a plugged cat. How is the power?

Thats what I was thinking. Also XJs are notorious for developing cracks at the collector of the exhaust manifold.

 

[redveloce]

Thats what I was thinking. Also XJs are notorious for developing cracks at the collector of the exhaust manifold.

They changed to a split cast manifold in 2000, which made the cracking much less common. I'd also expect the telltale ticking. The '00-01s have flawed head castings that commonly crack (I just replaced the head on my '01), but I haven't heard that affecting mileage. It tends to manifest through overheating, mysterious coolant loss, and dilution of the engine oil... All the same, it's worth a quick check to see if there's any 'milkshake' in the oil filler or radiator.

 

[MoGas]

I was thinking cat, too. Is there a MIL?

 

[dstock]

Thats what I was thinking. Also XJs are notorious for developing cracks at the collector of the exhaust manifold.

Plugged cat thought here as well.

 

[GJStringert]

Oh now THAT'S and interesting idea. I never even thought about checking the cat. The power seems just fine, we have a friend with an identical Cherokee and when I ride in his I really can't tell a difference.

Ok that's good, other than the O2 sensors I pretty much ruled out exhaust, so I'll pull it in and give it a thorough look as soon as I can.

MoGas, I'm assuming by MIL you mean Malfunction Indicator Light? No lights are on (except Low washer in the picture haha!), and no codes, at least not that Advanced Auto parts could read for free.

I can't imagine this is related in ANY way, but it does have a bit of an oil leak at the front of the crankcase, but I've heard the 4.0 is kind of known for that. I seriously mean a BIT, she can run oil change to oil change and never have to top off oil in between, it may by like 4 drops total per change, something I'm not really worried about...

Thanks for the replies all, we really appreciate it. I'll tear into the exhaust as soon as I have time and update accordingly!

 

[Lee_N]

Other things to check that could affect mileage would be coolant sensor (keeps it in open loop) and tps. A bad tps can indicate the throttle is open more than it really is. Any of these could go out of spec without throwing a code.

 

[MoGas]

When you say "Labored starting", does it spin over at a normal speed, just taking longer to start, or it struggles to turn over?

Can you post a video?

 

[bat]

I never even thought about checking the cat. The power seems just fine,

If it is the cat the motor would have a hard time accelerating

 

[FredB]

Did you check the air pressure in the tires? If it's been in 4WD, that kills mileage. My kids 97 gets crappy mileage, probably 11 mpg in town. My 90, until I put the correct CPS in it(not available for 91 and up) got 10 mpg in town when it was stock, and it's a stick.

Might ask at NAXJA, you don't have to be a member to post.