Hesitation, then SURGE...

99 Jeep Cherokee.

Here is the scenario, at home I park my Jeep in the garage and always back it out, I never have this issue. At work I back into a parking spot, after work I start my Jeep, pop it drive and I'm on my way... sort of.

I have been getting a hesitation, like something is clogged, I am barely moving so I push on the gas a little more, little more, sometimes almost flooring it and then after about 5 seconds its like everything comes online and I'm thrown into my seat. After that everything runs normal.

Anyone have this issue before?
Thanks in advance,
Nuke

I have the same exact issue. It's something in the engine compartment. It almost sounds like it's choking... I clean my throttle body every so often, and I can tell the difference. It does it alot less. However, I doubt this is the solution....I myself will also be waiting for an answer

try re-seating the connectors on your MAP sensor. That's what seems to be causing my 96 to do that.

I've heard a lot about this MAP sensor, but to be honest I have no idea where it is located. Can anyone explain it's location? Again, '99 4.0L Engine.

Thanks
Matt

nuke said:

I've heard a lot about this MAP sensor, but to be honest I have no idea where it is located. Can anyone explain it's location? Again, '99 4.0L Engine.

Thanks
Matt

Click to expand...

Can't tell you for sure, because they moved it around over the years. On mine it is bolted to the driver's side of the throttle body. It has a small rubber elbow on the bottom that leads to the TB.

It's an $80 part as compared to the $20 one that was mounted on the firewall of earlier XJ's. I don't know if DC kept it on the TB from 96-01 or if the went back to the firewall mount to save money.

My jeep was having a similar problem, where there were dead spots in the throttle position sensor. I'm not sure if its the same as what youre talking about, it would lag and run crappy when the throttle was in a certain spot but then catch up if I accelerated more. I took off the throttle body and "rebuilt" it- new TPS, idle air control, cleaned the MAP sensor connector and its tiny oraface in the TB, also gave the TB a throrough cleaning and a new gasket. Jeeps running pretty good now, noticeable improvement in pickup and that driveability problem disappeared.

Mine does it as well. Especially first thing in the morning if I don't really let it run a minute or so before i take off. Really does it sometimes on hot start ups, takes off fine, lags, i'll floor it basically then it shoots forward out of the blue. Also, everynow and then in the mornings, my viscus fan goes retarded and stops spinning the minute i take off, comes back, goes away again, then comes back and spins much longer than it should. Only does that on a RARE occasion, and it hasn't in a long long time. Not too sure what thats all about. Also a 99 4.0L.

Don't know if it's *exactly* the same symtoms, but it sounds very very similar to an issue I am just resolving on my '96 XJ.

Bugs in my gas! (No kidding.) Evidently there is a bacteria that can and does live in tanks. It's waste product is jelly-like, and can wicked-bad clog your fuel system up. Read on...

I was having similar symptoms to yours, but they gradually worsened to where it had multiple-minute-long episodes of absolutely awful running. I eventually got a check engine light on from it. Scanner told me multiple misfires and misfires cylinder 6. (Codes 300 and 306)

So, I started with the spark side- I already had brand new wires, plugs, and coil - done chasing the problem before I got the trouble light. Replaced the cap and rotor (which looked a bit carbon-y, so I thought it might be causing the misfires). Still had the issue.

My MAP looked in OK shape, cleaned and reinstalled. breathers and vaccuum hoses checked out, too. No effect.

Had the injector rail and injectors power-cleaned at a shop, since it had 120K and had never been done (a likely suspect, I thought...). Still NG. Good to have it done anyway, but still a bummer to not have it fix the issue.

Got a new fuel filter and swapped it, hoping against hope that i didn't have to chase the problem to the fuel pump (which is a major pain in the a*! to swap out).

Lo and behold, the filter was seriously clogged, and I had found the throttle bog culprit.

But: I had LOTS of clear jelly-like clots in my fuel filter. Some normal sediment also, but primarily this goo. I was baffled. "What the f..?" I said, and I was off to the magic of Google on the PC.
Check out: http://www.pfs-pros.com/page26.html

No joke: Bacteria in my gas.

I've just today added a different bug enzyme product than the link above goes to (the link just explains the bug thing well), and already the new filter and this bug-stuff are making a big difference. No more lag-then-surge behavior!

I'll post a review of the product I used once its been in the tank long enough to take effect. Probably under a new thread though. Bacteria as a search term will find it.

Aw, crap. Sucks to have to do this...

The gel in the fuel filter was not the problem. I have still got the initial symptoms.
So, disregard my last post as a fix.

Still, its good to know about the bugs in the fuel (I did actually have bacteria in the tank, it just wasn't what was causing the problem, I guess.).