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JeepJimNL 03-07-2012 08:40 AM

Why oh why is my Jeep so thirsty? (and other shenanagins)
Hi everyone.

I am a new member, and also new to the world of Jeeps.

Not only am I new to Jeeps, I am new to mechanics in general, so please forgive my crude & non-technical descriptions of what I think is going on. However, I am keen to learn, so any help in educating me would be very greatly appreciated :)

A week ago I bought an '89 4.0 i6 YJ, and it's INCREDIBLY thirsty... I'm talking less than 8 MPG in US & about 9 MPG in UK measurement.

Considering it's *supposed* to do at worst 16 according to Wikipedia, I figure there's something not right. Also, it HATES running cold. It takes a good 5-10 minutes to warm up on the auto-choke, and if I try driving before that it will not tick over. Also, the auto-choke stops working the second I touch the gas pedal - is this normal?

I pulled the carb apart with my Uncle, and we noticed that the flap over the hot air intake has been held open with a couple of screws by a previous owner.

Has anyone seen this kind of thing before and know why?

How should I proceed? Is this a garage job, or can I just adjust the carb screws myself from a guide or so?


brent in az 03-07-2012 09:41 AM

Welcome. Sounds like the previous owner forced the butterfly plates open as an attempt to compensate for no choke (I'm guessing). They should operate independently so just restore it.
Hang in there kid, I haven't had my coffee. Poor MPG can be cured but there's a lot to check. I can count 3-4 threads in the last week about this subject around here including mine called "Excessive fuel consumption"---and I got great advice. It's either the fuel system or ignition system and a lot of checking/ruling out.

banks316 03-07-2012 09:42 AM

Well first off you dont have a 4.0L you got a 4.2L. You wont get 16 its just not going to happen. You should get close to 14 hwy/12city
As for fuel milage need little more info to help

1) What size tires and gears you running?
2) If lifted is your spedo correct?
3) How you measuring your mileage (if spedo is off by little bit can affect your odometer)
4) Have you done full tune up (plug, wires, cap, rotor)
5) Is your Cat plugged (check for excessive heat)

JeepJimNL 03-07-2012 10:06 AM

Thanks guys : )

@brent - Thanks - I will check your thread later (at work atm). I will also try popping the screws from the previous owner out and see what happens.

@banks316 - I am running oversize mud BFG's. I don't think it's been raised (already had tires hitting the mudgards coming off of big bumps too fast). My speedo is NOT correct - but I've been running off the TomTom and it's reading about 10% below reality. I'm measuring mileage the same way. Are you sure it's a 4.2? My registration docs say 4.0! I haven't done a tune-up yet. I'll try to find time this weekend with my Uncle.

For the rest, I'll show this to some more mechanically minded people (along with the other threads you mentioned brent).


DirtyRob 03-08-2012 07:55 AM

From the factory Jeep didn't put the 4.0 in until 91. Before then it was a 4.2 which is carbureted. The 4.0 is fuel injected, and has more h.p. and better on gas.
It sounds like you may need to take you carb a to an "old school" mechanic that you trust, or get a new one if you do not want the headache. A lot of mechnics now days don't mess with carbs much

Xpress 03-08-2012 02:23 PM

Spark notes:

4.2 = carbureted

4.0 = fuel injected

Basically, your carb is WAY out of tune. The factory carbs basically just sucked and get worse over time. You can always upgrade to a Howell TBI fuel injection kit (~$1200+, you had better love the thing) which will get rid of carburetion problems entirely (cold start issues, rough idle, poor mpg, etc) or you can bolt on a weber carb which will also be a big improvement.

But getting a carb in tune is the biggest thing. If it's out of tune (from the wrong jets to wrong mixture settings) your engine won't run right and you'll be dumping gas down the road.

IAM747 03-08-2012 08:03 PM

Ok I can feel your pain. I have an 88 Wrangler and bought it knowing it had some low fuel milage issues. So to say the least I am somewhat hell bent on finding the cure. I started with only getting around 7 miles to the gallon. I am currently getting 11.5 per gallon.

Some quick stats.

31 x 10.50 tires no lift
4.2 Litre 258 AMC engine
Weber Carberator
A 904 Auto transmission
Stock gears.

Things I go from 7 to 11.5 mpg

My fuel filler hoses and vent hoses where you put gas in were dry rotted. Obviously by the stain coming below them they were losing fuel. (After pulling them out I am surprised they held fuel at all.

Most of the fuel lines that were rubber were dry rotten as well.

Carberator adjustment was a major factor.

Timing was not set correctly and was way off.

Newest and not yet tested for new milage...

Now for the latest I found, the front axles are locked in because of a faulty vacuum actuator basically I am driving with extra drag in the from and damaging the transfer case (LOL I know this because I just had to go into it and replace the drive chain and Hub range shift fork pads.

I wish you all the luck. I hope some of my findings help you.

Master4th 03-08-2012 08:34 PM

I did a Royal Purple swap in my driveline, I was amazed at the difference, the year before I had replaced front and rear diff, tcase, trans with regular high end lubricants. With the Royal Purple there was far less drag and things felt very very smooth.

I've also done this swap in my 04 Expedition. It was so noticable that my wife asked me why I was braking so hard, thats how much extra momentum was created from the RP in place of Motorcrafts synth.

Its not often that a product comes a long that really keeps its word.

Anyway, the Drag reduction from the lubricant increased my Expedition 2ish mpg, and when your only pulling 11 thats a big deal.

In my Wrangler I was seeing 6 to 7mpg after RP swap I was getting 9ish[Street] 11ish[Highway] increase.

JeepJimNL 03-09-2012 06:14 AM

Thank you so much guys.
Well, my beast must be a 4.2 then since it is definitely carburetted.

Very odd that they reg docs should be inaccurate on this point. I am in Europe... I don't suppose it's possible that the '89 models over here got different motors to the US ones?

Anyway, I have just found a US specialist in my town. I'm going to ask them if they can tune the carb for me and replace all the leads & lines. Hopefully that will go some way to curbing the thirst.

Thanks for the recommendations of the after-market carbs. I certainly can't afford to blow another $1200+ on it right now, but perhaps I can get something cheaper as a temporary measure. I'll check out the Webers.

Thanks again for all the advice :thumb:

JeepJimNL 03-09-2012 06:17 AM

I just checked eBay and found this Weber carb kit:

Weber Carburetor Kit Jeep Wrangler & CJ7 4.2L(258) Carb | eBay

Is this what I need? The price is manageable, not sure if it's reasonable for this product though. Still, if it gives me a few extra MPG it will pay for itself in no time!

JeepJimNL 03-09-2012 07:45 AM

OK, did some more reading over lunch, and found this great thread:

Then I went on eBay and found what seems to be the guy they were talking about above:

Motorcraft 2100/2150 JEEP/AMC/Eagle/Pacer Carb Upgrade 258/4.2 1.08 venturis | eBay

Before I pull the trigger, is this the carb to go for?
And should I ask him to fit it with an auto or manual choke? I guess auto is easiest in terms of the amount of work, but I have heard people say auto-choke sucks.

Cheers :-)

JeepJimNL 04-01-2012 04:32 PM

I've taken the plunge
After unsuccessfully trying to find a garage that isn't scared of carburettors, I've decided to go for the Motor Craft upgrade.

I've gone to Michael (ehrmannmp) on eBay since he is the man recommended on many forums.

I've ordered:

The MC2150 carb
The TFI distributor
An air filter kit to fit the new carb
A manual choke conversion kit

The whole lot cost $485 (375 euro) including delivery to the Netherlands. This is a lot more than I hoped to spend, but it's only a couple of tanks worth at current petrol prices, so if I get the kind of improvements others have seen it should pay for itself in no time.

It sounds like this is the best setup going for the 4.2 AMC (at least without spending an absolute fortune). I'll post progress and results here.

Based on what I've read I'm hoping for around 16mpg. Hopefully this isn't over optimistic.

Thanks again for all your help :thumb:

busemans 04-01-2012 09:19 PM

it is, other then fuel injection

SandyMushYJ 04-01-2012 10:13 PM

Let us know what you think of the new setup, Jim.

JeepJimNL 09-16-2012 04:18 AM

It all went on OK, then I Nuttered it, and it goes great.
Much faster pickup, and a more usable 5th gear.
It's still thirsty as hell though - I guess that's what you get for buying a 4.2.

If anyone knows where I can find a good guide on tuning the MC2150 I'd greatly appreciate it.

batty505 09-16-2012 08:37 AM

Hey jeepjim
My other recommendation would be to look at Fuel injection kits, for the 4.2l
There's a book called the jeep owners bible. Great read explains the different makes models.
Hope this helps

0III0forlife 09-16-2012 08:38 AM

A MC carb or any other for that matter, should be adjusted to the most optimum amount of vacuum that you can get out of it.
You have to get yourself a vacuum/ pressure gauge, and hook it to a main vac. line coming off the bottom of the carb. 18 is a good number to shoot for, but i have seen them run as high as 25.
start with 2 turns out on both of the adjustment screws and go from there. Go a 1/4 turn out at a time until the vac gauge starts to drop. then turn it back in just a touch.
You can set your timing the same way, or with a timing light. 8 degrees BTDC is where you want that to be.
Idle speed should be between 7-800 PRMs, closer to the 800 side is a good point.

Once you get it all in tune, you can expect a mileage of 12-16 or thereabouts.

I have an 87 4.2 with an AX15 5spd, running 33" rubber and mine gets 18-20 religiously, but, i have a few more upgrades done to mine, than what the "normal" person would do.

Ianbeaulieu 09-16-2012 09:46 AM


Originally Posted by 0III0forlife

I have an 87 4.2 with an AX15 5spd, running 33" rubber and mine gets 18-20 religiously, but, i have a few more upgrades done to mine, than what the "normal" person would do.

Would you share your ways of what you did to your Jeep to get that amount

JeepJimNL 09-17-2012 04:18 PM


Originally Posted by 0III0forlife (Post 2797144)
A MC carb or any other for that matter, should be adjusted...

Thank you so much for the advice!
I'll be doing this at the next opportunity (once I fix the horrible noise coming from my bell housing) :whistling:

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