Topic: hard time maintaining highway speed

[stan98tj]

98 TJ-2.5 l 85K- so the new head is on...and she starts up right away and all..almost there! took her out for the first time today on an extended highway trip and i noticed something odd....seems to be dogging/lagging real bad and im having trouble maintaining highway speed. the slightest hill or incline drops my speed down from 55 to like 45-49 and while my foot depresses the gas, i notice nothing happening, when the gas pedal reaches the floor there is like a 3 sec lag before it shifts and actually accelerates. def wasnt like this before i replaced the head...it could always do 55-75 easy. also, my gas consumption is down to 9MPG!!! certainly odd and im so close now i really just want to square off all these little problems that occur. Im running 31inch tires and no lift by the way....but that size should affect mpg nor power all that much.

[aw12345]

your milage is way to low with 31" tires it should be somewhere near 18 mpg. You sure the fuel pressure regulator, injectors and map sensor are working correctly also ignition timing set where it belongs and does it advance when you rev it up?

[oldjeep]

Did you wind up having the block checked by a machine shop?  This is the motor that grenaded a piston correct?     What all did you replace and what macchine work was done.

[stan98tj]

yes same engine with blown piston. wet compression test i did indicated head was cracked..so i bought a new one-not reman.  $$ was kind of tight so i couldnt have the block checked out-though i was pretty sure it was ok and that all that happened was a cracked head and blown piston. starts up 100% better than it used to and compressions are all the same and right on the money-150psi.  Jeep dealership assured me my fuel pump was fine..but now i dont know who to trust cause the first mechanic told me the pump was shot-and wanted $850 to replace it...and on top of it he broke my hood latches and didnt wanna own up to it!
I did check the spray pattern on the injectors and they were clean and sprayed nice.  Guess i should check fuel pressure this weekend? and perhaps prepare myself for a $400-600 job?

also, both mechanics (jeep dealership included) told me my sensors were fine...still...i dont know who to trust anymore-figured a jeep dealership would know more about jeeps than i do right?  I thought timing was locked in on the wrangler??  guess i could throw the timing light on it and see what happens...it seems to dog in the 50+ mph range..long lag and then it shifts if i am flooring it. and yea....9mpg isnt good.
any ideas??

[oldjeep]

Did you do a leakdown test on it?

As far as the fuel pump being a  $400-$600 job, that's insane. The pump is $93 if you just replace the pump.
http://www.partsamerica.com/ProductDetail.aspx?MfrCode=CTR&MfrPartNumber=P74186&PartType=52&PTSet=A

Figure 3 hours to replace by yourself, dealer should be able to do it in 1.

[stan98tj]

im not quite sure what a leakdown test is. ive been told that the pump cannot be replaced by itself rather the whole unit needs to be replaced. a friend suggested that the o2 sensor may need to be replaced and could cause this problem. can anyone confirm/deny this?

[jfrabat]

im not quite sure what a leakdown test is. ive been told that the pump cannot be replaced by itself rather the whole unit needs to be replaced. a friend suggested that the o2 sensor may need to be replaced and could cause this problem. can anyone confirm/deny this?

I replaced the fuel pump on my YJ in about 3 hours, and it was not so expensive (mine was around $110 with shipping included).  The only real PITA is dropping the tank, but I did it by myself with some help from the wife.  No rocket science involved!

As for the O2 sensor, I understand that the TJ has 2 of them (not sure, though, as there is only 1 in the YJ).  I replaced mine (about 5 minutes) with a new one (Bosch, about $45).  This one is really a piece of cake, so if you think the O2 sensor is at fault (it does not sound so to me, but that is your call) I would replace it.

By the way, I am getting 15 mpg on 31" tires on my 2.5L YJ if it helps any...

Felipe

[VA_YJ]

Have you checked to see if any DTCs are stored in the computer?  Not all codes trigger the check engine light.

[stan98tj]

DTC? im assuming thats an error code sent to the computer. ive been under the impression that you cant read that unless the light is on. according to the jeep dealership my sensors are fine....but then again they dont seem to know everything. i just want to know why its lagging and down to 9mpg. and id like to check and see if the O2 sensors have anything to do with it...the truck has 85k and they are original. other than that i have no idea what the hell is goin on

[aw12345]

leak down test involves putting compressed air in the cylinders and then you basically compare the pressure that you put in to the pressure that the cylinder holds ie the amount of pressure that leaks of is a percentage of that anything more than 10% is no good. I think your simplest check to see if the mechanicals are where they are supposed to be is hook a vacuum gauge to manifold vacuum. Should be somewhere close to 21" of vacuum at idle.
If not look at timing vaccum leaks restricted exhaust and so forth. With all the work you have done you haven't found the root cause of your original engine failure. Something is wrong and you need to find it or it will fail again.

[stan98tj]

With all the work you have done you haven't found the root cause of your original engine failure. Something is wrong and you need to find it or it will fail again.

well i think i have the original problem narrowed down. it always started hard from day one. with the new head, it starts fine. it always had low compressions, now with the new head the compressions are 150 across the board. I think the head had a minute crack in the head by the #4cyl when i bought it. The bad compressions were prob causing my pump to send fuel like crazy and at some point it shut down the #4 and combusted and blew a hole in the piston. The crack on the head is actually right under the #4 valve between intake and exhaust port-runs straight across.  Im not the original owner so i dont know if ill ever know what happened for sure-all i have is a guess. but what i do know is that 9mpg is bad on a 4cyl, and that it's lagging and not holding highway speed easily-when it used to be fine.

[aw12345]

9 mpg is artrocious, I don't honestly think the crack in the head was the problem. Coolant in a cylinder will bend the connecting rod. Low compression is hard on rod bearings but doesn't realy melt a hole in a piston.
Main cause for a whole in a piston is running lean on that particular cylinder or pre combustion. Ie pinging at high speed. Somehow the lousy gas milage is tied to your problem. Something like low intake vacuum, which would tell the map sensor that the engine is under a much higher load than it actually is, which causes the injectors to deliver way more fuel than necesary. Cause for this ignition timing, cam timing/loose or jumped timing chain or a flat cam even and last but not least exhaust back pressure. All of this can be pretty well diagnosed with a vaccum gauge or a combo vaccum gauge fuel pressure gauge the latter is even better since it you can use it to measure exhaust back pressure. Which you could measure by putting a fitting in the oxygen sensor hole and running it like that on the street

[stan98tj]

well my question to you is where would you start (please keep a limited budget in mind) and would you consider the O2 sensors as the problem or part of the problem?
also keep in mind jeep dealership tested my map sensor/computer etc and said it was fine..but i guess i cant really trust em

[aw12345]

Start with a vacuum gauge its cheap and will tell you where things are at. Say that the intake vacuum is low say 15" of vacuum instead of the 20-21" that is normal for a 4 cyl engine. This will cause the map sensor to read low vacuum/ higher engine load than there really is hence higher fuel consumption is the map sensor bad no. It just tells the ecm what it sees. My aproach would be to start as above if it turns out to be low, check ignition timing there are some factory manuals on here, follow the procedure, check the timing chain to see if its badly streched, if that doesn't solve it check the catalytic converter and muffler for restricion. if that restores vacuum its either fixed or then it becomes time to start looking at the data stream of the ecm. But in  my book its mechanical and most likely tied to either timing or exhaust restriction

[aw12345]

here is a link to the page with the correct factory manual

http://4bangerjp.com/forums/index.php?topic=414.msg1850#msg1850