1998 Cherokee 4.0 Auto Economy Issue
 

Hi all,

Sorry my first post (and likely my next few) are based around an issue, but I was hoping you guys may know the solution to my problem please.

I have a 1998 Cherokee LTD 4.0 Auto. It runs fine, but I have very poor economy and a less than perfect idle.

So far I have checked/done the following:

* Compression test (wet and dry) - no issues found although they weren't the best readings I've ever seen
* Replaced plugs (leads, cap & rotor recent)
* Removed, checked, refitted inlet manifold & Exhaust manifold
* Replaced 1st lambda (02 sensor)
* Cleaned throttle body & Idle Control Valve

My O2 sensor output is nicely fluctuating between 0.1v and 1v, as it should.

I also have a LPG (I think you call it propane) kit installed on the vehicle, which gives even worse economy. BUT if I adjust the propane system to make it idle richer (takes the O2 reading off the scale) it does seem to idle better, but still not perfect. There is little difference in idle between the petrol and propane unless I richen the propane mixture.

The trip computer states around 16.4mpg but from fill-fill I am getting no more than 12mpg on Petrol and 10mpg on Propane (which is correct as you get a upto 20% reduction in mpg on Propane)

I have checked the tyres, drive slower, turned AC off and more - but nothing seems to be bringing the economy up or improving the slightly spluttering idle

Any further ideas please guys?

It runs fine, seems to pull ok, sounds much better since I did the inlet/exhaust manifold check (maybe had a gasket leak), but this is taxing my brain a bit.

To add to the problem, I live in the UK so fuel is about $8 a gallon (US size) for Petrol and $4 for Propane.

I've converted all figures above to US gallons and MPG to aid your diagnoises and comparisons.

Thanks in advance guys. I look forward to hearing your suggestions.

Mick

Mick,

Ouch, more than double what I last paid. I use propane for domestic heat and put 400 gal in my tank @ $1.87. Gas is currently $3.59 where I live.

Not likely to be your problem, but on my 1997 the wiring harness was chaffed and shorting causing a misfire which drastically cut the mileage. See thread: https://www.cherokeeforum.com/f2/new...-32066/index2/

Best of Luck :)

Is it sputtering and cutting out randomly? Check engine light? Did you notice any cracks in the exhaust manifold when you took it out?

I had this issue until I pulled out the exhaust manifold and found 2 big cracks, common issue. I replaced it with an aftermarket header and MPG was much improved, check engine light cleared, all good!

Hate to say it. But you don't buy a Jeep XJ for fuel economy. It just isn't going to happen. I average between 16 - 17 mpg. Some on he do get a little better but most of them are bone stock.

I'm curious. Why would a crack in the exhaust affect MPG? Does the crack influence the O2 sensors?

I removed the inlet and exhaust manifold for the purpose of checking for leaks and cracks etc. Sadly the exhaust headers have been replaced (before I got the car) and are perfect, as was the inlet manifold. I was hoping for an easy fix there, but there was nothing to fix.

This is the problem, everything I check is perfect, so why the less than ideal idle and the exceptionally poor MPG figures?

Someone suggested a crank position sensor for the idle issues. Any views on that?

I'm starting to think about removing the head and having the valves machined as we all know propane lacks the lubricating properties of Petrol and therefore can (often does) wear the valves seats and reduce compression.

How fussy are these engines with regard excellent compression? Some engines noticably suffer from poor sealing more than others.

Thanks for your suggestion though guys. Keep it coming please.

A crack would/could/does effectively mess up the efficiency of an engine in several ways, including the following:

1/ It will unbalance the escape of exhaust gases if the crack is on some cylinders and not others - therefore lossing scavaging and associated torque gains

2/ It will suck in air, resulting in an unaccurate reading from the 02/Lambda sensor(s).

3/ It sounds aweful!!! :whistling2:

4/ Back pressure is reduced, which most modern engines rely on for their torque output.

An exhaust system should be as sealed as possible - for safety too.

Howdy over there!, (and thanks for converting all that). Being as you are being so through there are a coupe more things you might want to conciser. The MAP sensor scenes vacuum, (how much you have your foot in it), and your PCM uses that to make a stab at roughly how much fuel to dump in there. (via injector pulse width). The other sensors, an finally the 02 put the final touch on top of that first guess. Any and all can be suspect, for instance my coolant temp sensor only drops to 300+ ohms at temp, where it probably should drop to closer to 200. (I barely cleared 20MPG last highway trip. I might improve a tad with that corrected) Those are my specs for a much older Jeep, I don't know yours right off.

The map is important. There are easy voltage checks with a DVM. More involved is to hook to a vacuum pump, and use an analogue meter to see the needle sweep and actually see the MAP operation. A similar test can be done on your TPS, seeing the sweep as the throttle is depressed. Not likely your mileage issue, but I've seen posted the TPS can affect idle. Maybe just a simple voltage test there?

Also here in post #2 there is stuff on the HO 02 sensor. Honestly I haven't even really read it....http://www.naxja.org/forum/showthrea...ht=sensor+test
OK, he posts the same readings you see, and adds that you shouldn't see over .4-.5 at 2000 RPM

So theirs that! Cheers.

buy 6 measuring cups, remove fuel rail and check fuel delivery.

02 is suspect. get that leak fixed. also check to make sure you don't have a vacuum leak. if the header/intake gasket is crumbling you'll have both a vacuum and exhaust leak. that absolutely will kill the mpg, performance and idle.

What were the compression numbers?
You can tell quite a bit about valves with a vacuum gauge, I'd toss one on and see what you have. Interpretation:
http://www.ifsja.org/tech/motors/vacuum.html

All the above has been done and no issues found. I have not yet checked the vacuum readings, so will dig out the gauge this week.

HOWEVER, things have improved and I may have cracked it. I have a another 350miles to do this weekend which will paint a better picture, but for now, things do look a lot better

Here is my log: https://www.fuelly.com/driver/dragpa...kee?fu=3080881

what were the numbers on the compression readings?

what do you mean by less than perfect idle? fluctuates at idle or idles at low rpm like 500?

Can't be too sure as my compression tester can't be trusted, but a known good engine provided a bench mark figure of 95/115 (dry/wet) and the Jeep provided figures ranging from 80-90 dry and 90-105 wet, so not perfect, but certainly nothing to worry about.

Incidently, the issue seems to have been fixed now. I managed 15mpg around town last week, and even managed a 23mpg on a run (these are US mpg figures) - but all this was whilst running on Propane, so half the price of petrol here.

This was great until a spark plug wire came slightly adrift and my economy dropped to 11mpg without me noticing.

So my last issue is the idle. I will make up new spark plug leads this week and see if that helps otherwise its back to the drawing board.