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Old 05-29-2007, 05:08 PM   #1
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lack of fuel?

ok i had my motor rebuilt and pretty much stock except it is bored.06 and have 4.0 throttle body and cold air intake. my problem is it seems my jeep doesnt get the fuel it needs. it seems to run smooth and on a flat or going downhill its not bad at all. the problem is it doesnt wanna pickup speed at all going uphill and even on flats it just doesnt seem right either although it does pickup smoother. i have the standard 5 speed and at time of motor rebuild i had a new clutch kit put in and also a new fuel pump. i also just recently replaced the fuel filter which helped out a bit but its still not what it should be. i did the 4.0 tb mod after the breakin and motor rebuild and that at least judging by drivability has helped. i just dont know what to change thats going to fix my problem. theres got to be a comp somewheres right? could this be a case of the comp not giving motor enough gass and require a new comp or burned chip for mine? the rebuilder used the original fuel injectors he said he cleaned them out good and such could bigger injectors help? i got a new sending unit and fuel pump from ebay and the ad gave it for years of i think 85 to 95 could this be a pump for the carberated engine and not be flowing enough for what i need? i really dont know where to look or how to tell what the problem is. i could just replace stuff and see what works but $ tight and i got break work on my car that needs done by end of june so dont wanna throw any $ at useless things. the idle is also a lil funky sometimes it idles rough and other times it idles fine. this really had me dragged down as the jeep was so much fun to ride in and now its like only way i have any power at all is over 3000 rpms and then the motor really seems to do well and speed really picks up after that but in 5th gear i cant seem to get over 3000 rpms and only at maybe 70 mph so as soon as i hit a slight incline its like hitting a brick wall so any thruway driving is almost out of luck as the norm here in ny is 75-80mph. (i know speed limit is 65 but ya do 65 to 70 and its like everyones trying to run ya over) now i know my jeep wont do the 0-60 in 4 sec and thats not what im looking for but it shouldnt be this hard to get up to speed and i shouldnt havta over rev it to get there niether. oh yeah and one other thing i got a 91 with the 2.5l 4 cyl.

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Old 05-29-2007, 06:05 PM   #2
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i got a 91 with the 2.5l 4 cyl.

There's your problem!

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Old 05-29-2007, 07:55 PM   #3
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yes i know maybe sometime in the future a 350 will be in the works but for now i havta make due with what i have and the 2.5l had enough balls to go and drive decently before the motor rebuild. rebuild motor bored .06= less power? somethings fishy there and i need to figure out what it is so it can be corrected and make it to the future chevy 350 conversion lol. if this dont get resolved im affraid its bye bye jeep. i wont sell my vette and i need something reliable as a dd during the week and winter if jeep wont cut it then it has to go. but i really really really wanna keep the jeep. ive wanted a wrangler ever sence i can remember and i love driving it so im going to do what i can while i can to keep it and make it better.
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Old 05-29-2007, 09:02 PM   #4
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wouldnt those computer chips do the job in adding fuel? I dont have much experience w/ obd 1 systems but i know that as a fact there are little resistors that you can add on obd 2 systems where it will read all the air entering the engine as cold there fore adding more fuel to make up for the "cold" air entering the engine.
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Old 05-29-2007, 09:10 PM   #5
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yes i know maybe sometime in the future a 350 will be in the works but for now i havta make due with what i have and the 2.5l had enough balls to go and drive decently before the motor rebuild. rebuild motor bored .06= less power? somethings fishy there and i need to figure out what it is so it can be corrected and make it to the future chevy 350 conversion lol. if this dont get resolved im affraid its bye bye jeep. i wont sell my vette and i need something reliable as a dd during the week and winter if jeep wont cut it then it has to go. but i really really really wanna keep the jeep. ive wanted a wrangler ever sence i can remember and i love driving it so im going to do what i can while i can to keep it and make it better.
Sorry about the smart ass answer. If I were you I would be looking at what the compression is. With the over-bored cylinders it is possible to not have the right size rings on the pistons and get a lot of blow by. That will kill your power under strain (like driving up hill) and will make using 5th nearly impossible. The other possibility is that you simply don't have enough spark to properly detonate the amount of fuel you are dumpin into it. If you still have the 20,00 volt system you might consider upgrading to a 30,00 volt system with plugs gapped at .045 instead of .035 for better burn on the gas going in. You would also need 8mm plug wires.

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Old 05-29-2007, 09:20 PM   #6
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Originally Posted by scooter18155 View Post
rebuild motor bored .06= less power? somethings fishy there and i need to figure out what it is so it can be corrected
You bored the cylinder walls which lowers compression ratio.. the lower the compression ratio the lower power you will have. The only reason i'd bore the cylinders is if i'm boosting the engine with turbo or SC and am going to run lots of boost (psi/bar). Boring the cylinders on natural aspiration engine will lower power every time UNLESS you add higher compression pistons at which point you will get a good gain.

If you bore the cylinder walls on a boosted engine you will be able to run higher boost since it wont knock because of too high compression.

so all summed up from above..

bore cylinder walls on NA engine.. lower hp
bore cylinder walls on NA engine + higher compression pistons.. higher hp

bore cylinder walls on boosted engine .. no psi increase.. lower hp but safer
bore cylinder walls on boosted engine + psi increase.. higher hp
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Old 05-30-2007, 03:41 PM   #7
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Originally Posted by Grandpa Jeeper View Post
Sorry about the smart ass answer. If I were you I would be looking at what the compression is. With the over-bored cylinders it is possible to not have the right size rings on the pistons and get a lot of blow by. That will kill your power under strain (like driving up hill) and will make using 5th nearly impossible. The other possibility is that you simply don't have enough spark to properly detonate the amount of fuel you are dumpin into it. If you still have the 20,00 volt system you might consider upgrading to a 30,00 volt system with plugs gapped at .045 instead of .035 for better burn on the gas going in. You would also need 8mm plug wires.

That's the best I can do long distance!

the answer you gave was ok its probably what is part of the problem i do wish i could convert it its just not that smart to do financially right now. the electrical issue is a good idea i may look into doing this as i havent thought of it and it probably could use a good tune up anyways.
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Old 05-30-2007, 03:49 PM   #8
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Originally Posted by Triple88a View Post
You bored the cylinder walls which lowers compression ratio.. the lower the compression ratio the lower power you will have. The only reason i'd bore the cylinders is if i'm boosting the engine with turbo or SC and am going to run lots of boost (psi/bar). Boring the cylinders on natural aspiration engine will lower power every time UNLESS you add higher compression pistons at which point you will get a good gain.

If you bore the cylinder walls on a boosted engine you will be able to run higher boost since it wont knock because of too high compression.

so all summed up from above..

bore cylinder walls on NA engine.. lower hp
bore cylinder walls on NA engine + higher compression pistons.. higher hp

bore cylinder walls on boosted engine .. no psi increase.. lower hp but safer
bore cylinder walls on boosted engine + psi increase.. higher hp
yes but the pistons were ordered from summit for the oversize bore and if the oversized bore pistons are for lets say 9:1 compression then that should be your compression with a stock head compression can be increased or decreased with head work and my head was shaved down a tad so compression should increase from the given rating. the guy who rebuilt the motor was told to stay with a compression as close to stock as possable but a lil more was ok as long as it wasnt getting too extreme. (by the way i dont know what compression pistons were used was just saying 9:1 as an example i think i may ask the mechanic to make sure)
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Old 05-30-2007, 08:04 PM   #9
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boring a motor does not lower the compression ratio, it will actually increase slightly, simply put you are increasing the volume of the cylinder and compressing it into the same space as before (cylinder head volume and head gasket) . If all that was done was bore and rering without replacing pistons or cylinder head the compression ratio probably went up nominally.
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Old 05-30-2007, 08:15 PM   #10
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i got a new sending unit and fuel pump from ebay and the ad gave it for years of i think 85 to 95 could this be a pump for the carberated engine and not be flowing enough for what i need?
I am not positive on the changeover year but I think it was in 89that they went to fi, you might want to check it as a carburated engine requires less fuel pressure than a injected motor
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Old 05-30-2007, 09:16 PM   #11
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boring a motor does not lower the compression ratio, it will actually increase slightly, simply put you are increasing the volume of the cylinder and compressing it into the same space as before (cylinder head volume and head gasket) . If all that was done was bore and rering without replacing pistons or cylinder head the compression ratio probably went up nominally.
the pistons was changed out and the head was shaved slightly due to it was warped a tad. new valves new cam. other then the oversized bore everything was done to keep it as close to stock as can be. meaning it has a stock cam the head is stock (other then being shaved as prev mentioned) the crank was turned to clean that up and used oversized bearings. the ignition system is stock as well. i think that might be the first thing i look into as well as the fuel pump.
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Old 05-30-2007, 10:12 PM   #12
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Originally Posted by Scotty355 View Post
boring a motor does not lower the compression ratio, it will actually increase slightly, simply put you are increasing the volume of the cylinder and compressing it into the same space as before (cylinder head volume and head gasket) . If all that was done was bore and rering without replacing pistons or cylinder head the compression ratio probably went up nominally.
If you increase the space inside the cylinder so that it holds more compressed gasses but do nothing to help the motor compress those gasses, it will lower the output because less pressure will be built up during combustion. The same amount of fuel is being put in, the spark hasn't changed, the space available for combustion has increased=lower performance. If you put in more gas, or help combustion by replacing the distributor and coil to go up in voltage (longer, fatter spark) you could return to the performance you had.

I changed the distributor and coil to an HEI unit on my 88 YJ and it was much quicker with more hill climbing power. It also lowered the combustion gasses at the tailpipe.
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Old 06-01-2007, 03:55 PM   #13
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If you increase the space inside the cylinder so that it holds more compressed gasses but do nothing to help the motor compress those gasses, it will lower the output because less pressure will be built up during combustion. The same amount of fuel is being put in, the spark hasn't changed, the space available for combustion has increased=lower performance. If you put in more gas, or help combustion by replacing the distributor and coil to go up in voltage (longer, fatter spark) you could return to the performance you had.

I changed the distributor and coil to an HEI unit on my 88 YJ and it was much quicker with more hill climbing power. It also lowered the combustion gasses at the tailpipe.
hmmm like i said before i plan on jumping up the voltage. but am i misunderstanding compression here? i dont know what stock compression was but lets say 9:1 just for the convo. if i ordered oversized pistons at 9:1 compression shouldnt the compression stay the same considering there was no head changes even though the cylender is oversized? not trying to say your wrong just trying to understand this cause i told the mechanic to buy the pistons that was rated at stock compression. because thats what i was aiming for either stock or a tad more. i just didnt want him installing pistons that woulda given me too high of compression. dont forget installing the pistons at a stock compression ratio pluss having my head shaved should gave me slighly higher compression by my thinking. this is why i was thinking i am either lacking air or fuel into the motor to make it run right. i didnt even consider the electrical side until this thread so that probably be first thing as i am sure at least plug wires needs changing.
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Old 06-01-2007, 04:31 PM   #14
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i'm wondering if the split fire or those platnum +4s will do anything as that goes?

As the compression ratio goes the higher the compression the hp increases in the higher rpms but lowers in the lower rpms and i gues it should go opposite also..

stock 4.0 wrangler compression (tjs in preticular) is 8:1 according to cars.com

shaving the head = bore it out?

as the higher vs lowering the compression goes i was thinking that when you shave the head and bore the cylinders you make more space when the piston is at top dead center and adding same amount of fuel and a bit more air (vacume from the cylinder) to that you lean it out which decreases power and the compression goes down because now you have more space in there with less a/f mix inside.
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Old 06-01-2007, 05:37 PM   #15
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i'm wondering if the split fire or those platnum +4s will do anything as that goes?

As the compression ratio goes the higher the compression the hp increases in the higher rpms but lowers in the lower rpms and i gues it should go opposite also..

stock 4.0 wrangler compression (tjs in preticular) is 8:1 according to cars.com

shaving the head = bore it out?

as the higher vs lowering the compression goes i was thinking that when you shave the head and bore the cylinders you make more space when the piston is at top dead center and adding same amount of fuel and a bit more air (vacume from the cylinder) to that you lean it out which decreases power and the compression goes down because now you have more space in there with less a/f mix inside.

my engine is a 2.5l not 4.0 and your probably right about the fuel mixture and is my thinking as well that i was either putting same amount or less of gass into the cylnder and getting less power because of it. I dont know much about the comp in the jeeps so i was concerned with the comp not allowing enough gass into the cyl because it thinks the cyl is smaller then it is. it also has stock injectors so i was also considering changing these to a higher flowing injectors. a friend of mine has a code reader for just about anything that ya can scan. might see if im throwing any codes as well to see if anything pops up. not sure if yjs have this ability opr not but cant hurt to look and find out.
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Old 06-07-2007, 10:07 PM   #16
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If you increase the space inside the cylinder so that it holds more compressed gasses but do nothing to help the motor compress those gasses, it will lower the output because less pressure will be built up during combustion. The same amount of fuel is being put in, the spark hasn't changed, the space available for combustion has increased=lower performance. If you put in more gas, or help combustion by replacing the distributor and coil to go up in voltage (longer, fatter spark) you could return to the performance you had.

I changed the distributor and coil to an HEI unit on my 88 YJ and it was much quicker with more hill climbing power. It also lowered the combustion gasses at the tailpipe.
Sorry but you are not correct on this... the standard with anyone building motors, cubic inches = Horsepower....period.
There are exceptions to the rule, but that is do to design of heads, stroke, rotating mass changes. Buick V6 motors are quite impressive for there small size, and honda v-tech motors too make an incredible amout of power despite there small displacement.
But bottom line is that if you have a motor making 150hp on stoke displacement and you do nothing to it but open up the bore .030 or .060, you will make more power. Your new pistons that you would need due to oversized bore would be desinged to match the stock compression. They would not sell pistons for oversized bore and not correct any loss of compression. Unless desinged for a turbo motor.
I will agree that adding hiigh compression pistons will increase it much more than stock piston gains.
~Rich

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