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View Full Version : Poor Gas Mileage, Sputtery idle.


jeephick85
January 24th, 2008, 18:27
Hi All, I would appreciate a little help. I have searched on here for other peoples similar problems and there seems so be many different possibilities. The problem I have at the moment is poor mileage, a slight hesitation on take off which you can barely feel, and it has a sputtery ( 700 )idle which sound like a slightly misfire. it also doesn't kick down when im going 30 - 40 mph if i give it more gas. If I'm giving it a hard time forcing in the snow and let off the gas suddenly it will sometimes stall out on me. I have changed my plugs and wires ( 35 gap). I did all my vacuum hoses with new hose, have changed my oxygen sensor. I put the scanner and readjusted my TPS sensor, which is at zero and doesn't have a dead spot when tested. Could an intake or manifold leak cause the problem, it also has the catalyser removed.

87 cherokee 4.0
Thanks Alot, Jamie

89xj
January 24th, 2008, 18:35
test the egr valve. does it have a slight backfire or miss when you let off the gas pedal?

what do you mean by zero on the tps. i have a manual tranny and when i test the tps, i get just under 5v and the output is set to 83% of that, IIRC. havent done it in a while.

since you have a meter, you may want to spec out all the other sensors. if you dont have the procedures, go to www.lunghd.com search through the on-site tech info for renix sensor diagnostics.

intake leaks will give you a slightly high idle.

sputtering idle could also be a dirty IAC and throttle body

Marc w
January 24th, 2008, 18:37
test the egr valve. does it have a slight backfire or miss when you let off the gas pedal?

what do you mean by zero on the tps. i have a manual tranny and when i test the tps, i get just under 5v and the output is set to 83% of that, IIRC. havent done it in a while.

since you have a meter, you may want to spec out all the other sensors. if you dont have the procedures, go to www.lunghd.com (http://www.lunghd.com) search through the on-site tech info for renix sensor diagnostics.

:looney:

89xj
January 24th, 2008, 18:43
:looney:

i added a few things to my post but what was :looney: about it.

jeephick85
January 24th, 2008, 19:49
Ill for sure check out the other sensors. I checked MAF sensor and it was responding properly, and i recall checking most of the other sensor signals and all seemed right. I set the TPS sensor at 0% when it was turned off, ignition on, i misplaced it at one point and it was at 5% and the engine idled to high so I put it back to 0 %. What could cause the tranny not down shifting. Also if im on the highway at 60 MPH its reeving at 1600 in OD and 2300 in 3rd. Automatic trann..
thanks alot, Jamie

jeephick85
January 24th, 2008, 19:52
Forgot to say that it does backfire when its decelerating and it seems to miss when it idles, when its revving it seems to be fine. Jamie 8 miles to the gallon

jeephick85
January 25th, 2008, 12:41
HI ALL, as of today it only heats up when im driving cooler air tends to blow out when idling.

keep0njeepin0n
January 25th, 2008, 12:54
Ill for sure check out the other sensors. I checked MAF sensor and it was responding properly, and i recall checking most of the other sensor signals and all seemed right. I set the TPS sensor at 0% when it was turned off, ignition on, i misplaced it at one point and it was at 5% and the engine idled to high so I put it back to 0 %. What could cause the tranny not down shifting. Also if im on the highway at 60 MPH its reeving at 1600 in OD and 2300 in 3rd. Automatic trann..
thanks alot, Jamie

Your 87 does not have a MAF sensor....what did you check?

jeephick85
January 25th, 2008, 13:04
Sorry meant to say the MAP sensor up on the upper fire wall.

keep0njeepin0n
January 25th, 2008, 13:10
Well your RPM's at highway speed seem right to me...were you asking if they were wrong?

Also, you say you've changed the plugs and wires....what about the cap and rotor? Also, check the coil...I had an ignition problem with mine and come to find out, the coil had a crack in it.

jeephick85
January 25th, 2008, 13:30
I didn't change the cap and rotor and I haven't checked the coil either.As I wrote before it only has a smooth idle but it seems to spit and backfires sometime when i release the accelerator. My mileage is at 8 MPG in the city. Also for the tranny what would make it not want to down shift when i press on the gas, I have to floor it to have a down shift when Im going about 30.Thanks

keep0njeepin0n
January 25th, 2008, 13:31
Ok, so check/replace the cap, rotor and coil!

jeephick85
January 25th, 2008, 13:38
Yes Sir. Im actually gonna go right now and get them and Ill post back on here tonight to let the news sing. Hopefully it will improve somewhat
Thanks

keep0njeepin0n
January 25th, 2008, 13:41
Well I wouldn't run right out and buy them...cap and rotor, yeah they're cheap and should be replaced so often anyways since they're so cheap and can cause ignition problems, but I wouldn't run right out and buy a coil without at least checking it first...they're not all that cheap!

89xj
January 25th, 2008, 15:48
backfiring on deceleration is usually a malfunctioning egr or intake manifold leak.

jeephick85
January 25th, 2008, 21:25
OK. I changed the Rotor and Cap, Plugs and Wires, checked the TPS for dead spots, Everything was OK. Now that I have changed the Cap and Rotor, I can easily hear it misfiring. I put all the wires back the way they were, and it idles smooth for about the first 2 mins and then it starts to go choppy. The idle goes from 700 to below 500 and the lights dim slightly, but hasn't stalled out yet recently. All I haven't done is change the coil. I know I keep coming back to my tranny shifting at low RPM's (1500). It always seems like its dragging, any time I go up a slight hill I loose speed significantly and if i give it more gas it continues the same and if i push it to the floor it will finally downshift. What is causing this??? I think my ole 85 2.8 had more nuts than my 4.0. at the moment. Thanks Alot Guys. Jamie

Ecomike
January 25th, 2008, 22:44
Your power, shifting and probably the rest of your problems are a bad TPS for sure. It should read about 0.73 volts on the ECU side of the TPS when properly set, positioned. The TCU side of the TPS is definately bad and causing your low rpm/shifting, low power,etc problems. I fought your problem for 2 years on my 87 before I finally figured it out. I have a blow by blow history, with special test methods and data in the "RenX Files" thread here, OEM forum. Go read the last 40 or 50 posts in it. Use advanced search for a title "ReniX Files, use Ecomike for the poster, and OEM forum.

Once again it is the back side, square connector on the TPS that goes to the TCU, Transmission Control Unit that you have not tested, that is bad. And while you are at it, check the ground from both sides of the TPS wiring harness connections to the battery negative post. They seem to go bad too!

jeephick85
January 26th, 2008, 06:56
Thanks Buddy, I remember i checked out the TPS, I know it doesn't have a flat spot and i remember that when i set it the throttle position was at 0% and the voltage was a zero also if I'm not mistaking. Ill go down and get myself a new TPS sensor and hopefully it will be the end of these malfunctions. Will A BAD TPS cause it to misfire?? Thanks alot , Jamie

Ecomike
January 26th, 2008, 11:56
"Will A BAD TPS cause it to misfire??"

YES it can. TPS failure symptoms vary. It can cause a variety of different problems, depends on where it failed, and how.

jeephick85
January 26th, 2008, 12:34
OK just changed the TPS its not zero in yet, now what exactly am I looking for number wise when I adjust it. Right now its on seems to be shifting better but it still has a choppy idle.

Ecomike
January 26th, 2008, 19:31
OK just changed the TPS its not zero in yet, now what exactly am I looking for number wise when I adjust it. Right now its on seems to be shifting better but it still has a choppy idle.

Use the square connector, A is the ECU 5 volt input, B is the TPS return output to the TCU. D is the ground wire. With all power off, D should read less than 1 ohm to the negative battery post, ground. If greater than 1 ohm fix thwe bad ground wire conection, or clean the ground contact areas and run new ground wires. Then the B return wire when the TPS is connected to the wiring harness and with the power, ignition switch on, engine off, do not start it, test the return voltage (B to D) on the TPS, it should read 83% of the TCU input voltage (A to D), if not loosen the TPS mounting screws and rotate it until you get 83%.

jeephick85
January 26th, 2008, 20:02
OK perfect very much appreciated ECOMIKE and all the others. Ill go and do it now if i can manage to fix my ohm meter, if not tomorrow morning. jamie

jeephick85
January 26th, 2008, 20:54
I checked the reading for A to D with the ignition on and I'm getting 2.5 volts. As for the ohms I'm not really sure how to read my needle on this old meter. jamie

jeephick85
January 27th, 2008, 08:06
As from last night I have decided to change the ground wires and run new ones everywhere due to fact there was a small fluctuation in the round signal from various spots. Going to try and get the new TPS adjusted properly today. LET me have any advice!! Jamie

Ecomike
January 27th, 2008, 18:32
I checked the reading for A to D with the ignition on and I'm getting 2.5 volts. As for the ohms I'm not really sure how to read my needle on this old meter. jamie

OK, that 2.5 volts should be 5 volts, so now you get to trace that A wire back to the TCU (Transmission computer) to see where it is losing voltage. If you will search and read the transmission computer threads here for pin out data * on the TCU itself, and for its location, you should start at the TCU to see if it is supplying 5 volts. *I know its posted in several threads by me and others this past year already.

Never mind, found it, it's D1, a red wire is the 5 volt supply to the TPS!

jeephick85
January 28th, 2008, 15:29
Ok perfect ill check it out tonight, its running alot better, shifts better, accelerates better but I'm leaking around the manifold and being a one piece seal with the intake I think it might be playing games around there so im gonna change it tomorrow night. As for the catalyser should i put one back on, i took it off initially cause i hit a rock and it busted up the ceramic and jammed up my tail pipe, so will it run more efficient with catalyser? thanks, Jamie