The scenario:
Recently went to 4.88 gears, and a 4.0L throttle body on my 95 4 banger YJ with 33" tires. I have a problem with engine surging when in 4Low and 1st gear between 1500 and 1800 rpm. Aside from making for a rough ride, being annoying as all get-out, and rough on my drive train. Has anyone else had this problem and figured out how to fix it?

Next up on the project list is a header and 35" tires. I have been thinking maybe the 4.88's are a bit deep for the 33's, and that 35" tires will balance things out a bit. Or could it be I need to get the header installed since I now have much more air going into the engine?

On the plus side, the 4.0L throttle body and 4.88's rocked in Moab. Nothing like having the ability to throw more power and torque to the ground without building up a lot of speed!

I'm open to suggestions gang!

Uhhh....hmmm...thats a tough one

Low end throttle response is real good. Mash the skinny pedal and the little 4 banger comes to life. But.....This seems to happen when you're in a *cruise configuration*. i.e, neither accelerating or decelerating, and little to no load on the engine. Again, between 1500 and 1800 rpm. Above or below that, all is well. The problem even carries over to second gear sometimes.

Somewhere I'm forgetting something simple from Engine Theory and Performance 101!

well my Jeep, in 2wd on the street with kinda surge. If Im going down a slight incline and barely lay my foot on the gas it starts slightly surge...but It started doing that after the locker install so thats what I blame it on.

forgive me for not knowing, but what do you mean when you say the engine surges?

Sam

A rise and fall in engine power without any change in throttle positon. Kind of a *speed up-slow down-speed up-slow down* action. And in a similar condition as renegade described. Just barely any pressure on the gas pedal.

I've got one vote from a co-worker that I need to get the header installed to let the engine *breath out* better since I put on the larger throttle body.

I had a 99% stock 91 YJ with the 4 popper, it also surged pretty good too, I never figured it out, all I had on it was a free flow exhaust and 30x9.50's, it would surge in first gear while accelaterating and sometimes just a little in seconed gear. But I dont remember it doing it any other times.

I had the same problem. I put the 4 L throttle body on my 4 popper and it started surging. But it surged in all gears at all speeds and surged all the time unless I was at wide open throttle. Then I installed some pacesetter headers then all was well. My guess would be that I had a bad exhaust leak which made the O2 censor think the car was not getting enough gas and dumped too much gas to run well. I really had a bad leak because my jeep is QUIETER with the header on.

I think you just found the answer for me. My stock manifold has a big crack in the plenum area. We'll see what happens when I get the header on. It's been sitting here for a couple of months, I just haven't had the time to mess with it.
Thanks!

Dennis,

The 4.88 are not enough for 35's. 33's are perfect with 4.88's. I have the 4.88's with my 35's and 5th gear is not used to much. As far as rocks go, we'll the 4:1 does the job real nice.

zman

Dennis,
Go to your local jeep services shop. Pay them the $50 to hook your jeep up to an analyzer. Sounds almost like your computer needs to be calibrated for the throttle body. The report from the analysis should confirm or at the very least give you an answer.

Food for thought Dennis...
Your engine can breathe better, but what about exhaust? Most often, intake and exhaust mods should be done together...
Could also be TPS or a MAP sensor going bad. Tap the MAP gently and see if it surges on you. The TPS will have to be diagnosed by checking that it has the proper voltage values.Has it wrote a code to the computer?

Hmmm...I may do that RayRay.

Clem,
I had planned to do it that way. Things just didn't work out, so I still have a nice enw header sitting in the box that should have been put on at hte same time. You did hit one possibility with the MAP sensor. I have to wonder if maybe it's getting any residual oil film on it from running a K&N?

ZMan,
Thanks for the info! Before the 4.88's, I very seldom got to use 5th. And I don't mind spinning a few extra RPM either. I had a hunch that the 35's would put a damper on 5th gear again. I'm still trying to decide to do a TeraLow kit, or Atlas II. But that's way down the list right now.

But...I do have a nice new top with tinted windows now.

is the check engine light coming at all? and just out of curiousity what is your crawl ratio overall?

I have no had any issues with residual K&N oil in my map sensor.
Also, are you running a high flow cat or muffler? That will also help. You can do the header, but if you still have a botle neck, the header is worthless as far as performance gains...

Thanks Clem, The K&N oil was a long shot. But ya never know unless you toss it out there! Not sure when I'll get the header installed, Been way to busy. And now I'm changing jobs, and will be working 60 hours weeks for a bit. At least it's back to normal day time hours!

Check engine light is not coming on, and in all honesty, I have no clue what my crawl ratio is. Stock tranny and t-case, 4.88 gears turning 33's.

After installing the headers in my 4 banger the surging went away but a week afterwards the engine would run for about 15 minutes then shut off. I would wait a minute or two then it would start right up. It took me over an hour to drive 10 miles back home. The check engine light never turned on turned out to be the crankshaft positioning sensor.

Hope that helps

Thanks! Looks like getting the extra air out of the engine is the answer. Now if I can slow down long enough to get the header installed, and the exhaust reworked.