willd

Hey guys!

I've done a good amount of research but haven't been able to come to a conclusion. My jeep is acting up a little bit. Its a 1989 with the Renix 4.0. Stock gears, 4.5" RE lift on 31" tires. 142k miles and I bought it recently. Here's what I got going on -

-HORRIBLE gas mileage. I'm not expecting prius numbers but I'm getting roughly 150-160 miles a tank. The best I've seen is 180. I've aired my tires up to the proper PSI and from what I've been reading, everyone is thinking it's the O2 sensor. Anyone else have suggestions on what to check?

-On startup it will surge and hold it's RPM at 2k and will not lower. I've cleaned the IAC and it went away for a little bit. It seems to be getting worse as it did it yesterday and I had to shut it off 4 times before it went away. Could cleaning the TB be the solution to this? Is this also a symptom of a bad O2 sensor? I've also flipped the auto switch into POWER mode and it did help MPG a little bit.

-Twice (and only twice) the Jeep has taken a long time to crank and didn't start. I cranked it for probably 10-12 seconds before giving up the first time. The second time it probably took 30 seconds and 3-4 tries to start it. I'm thinking it's probably the fuel filter, as the current one was installed in 2009 and this Jeep has sat a lot since.

I've read Cruiser54's write ups and I plan on doing all of them over time and the Volvo injector swap. As far as I know the grounds are all fine and it looks like they were cleaned at some point by the PO.

Sorry for all the questions, but just want to get all my bases covered here.

Thanks in advance for the help!

willd

Alright just a quick update guys! I went ahead and replaced the fuel filter. The gas coming out of the old one was pretty gnarly. The RPM's still spike on startup but hasn't been doing what it did before where it would hold there.

Anyone have any suggestions on how to fix my other issues? Mainly the horrible MPG.

Batman33165

Go completely stock, if you want great MPG.

I also replaced my injectors but did not go the volvo way, actually spent some serious cash for mine. But i am lucky to get 110 miles on a tank in the city, i have not tried driving the jeep on the highway though so maybe it would be more.

*** I have a 87 cherokee, 35" tires, 4.56 gears, manual, and not sure on the lift but closer to 6.5 ( i think).

Jeepwalker

I'd probably start by checking for vacuum leaks. Mainly because it's cheap and quick and you can rule that out anyway. Plus hoses do get brittle and crack/degrade. That can cause high idle. What I'd do is disconnect the brake booster vacuum and cap the engine nipple where it connected to. Get some starting fluid or carb cleaner ...or even a propane tank with a 2' length of fuel hose works great. Start the engine and if there isn't any high idle you know it was the booster. If there is, go around with the carb cleaner spray or hose (connected to a propane torch - unlit of course). Spray or wave the propane around the vac hoses, around the t-body, intake, etc. Make sure your air box is connected ...you're not trying to spray INTO the throttle body, but at it's base, etc. If there's a vacuum leak it'll suck some conbustible in and should hear an engine speed or change in operation. Then you know you found something. Might want to disconnect and plug the vacuum hose to the canister too.

The IAC area could be really clogged up too behind it. Inspect it again. Could be the pipe that comes up from the exhaust, the EGR system.
I had a rodent carry an acorn which got lodged in the accelerator area causing high idle. Nothing seemed to fix it until I happened to see it.
Could be an o2 sensor. If it's high enough mileage, might be worth replacing anyway. I know they're not cheap, but their outputs do degrade over time/miles.
Wiggle all the connectors/grounds.

Get it to run smooth and right before you dig into the mileage. One might fix the other.

Let us know how it goes.

willd

Hey everybody!

I was finally able to fix the engine surging issue I had. The last time it surged it held at around 2500 RPMS. I popped the hood and unplugged the IAC, waited a few seconds then plugged it back in and it dropped. Replaced the IAC and it hasn't surged yet!