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Check Engine Light on During Acceleration

6K views 17 replies 8 participants last post by  fishadventure 
#1 ·
1992 Jeep Wrangler with 6 cyl in it, I have been getting poor fuel mileage. Im talking like 7 or 8 miles to the gallon but the jeep seems to be running fine. Idles smooth at about 850 rpms at idle and when accelerating there is no hesitation and i have done the following so far.

Changed Plugs, Wires, Cap, Rotor, Fuel pressure regulator, and cleaned the EGR Valve, and also cleaned the Throttle position sensor and have put a new O2 sensor in the exhaust (there is only one and it is before the catalytic converter).

When i am accelerating (it is a 5 speed standard) The check engine light comes on during acceleration once i release the gas and go to the next gear it goes out until my foot touches the gas pedal again. It will do this all the way up to 4th gear. Once i am in 5th gear there is no more check engine light issues whatsoever. I really cant afford to pay someone 75.00 to check the codes and have tried the key on/off thing but it doesn't seem to be working. So i guess my question is... How do i fix this or what could it possibly be?

Also my odometer is off by 10mph. When i am going 35mph i am actually only traveling at 25mph. I have 31in. tires on the jeep no lifts of any kind. Is this cause because of the tire size or could it be something that has to do with my crankshaft positioning sensor? timing? or what? this is all throwing me for a loop here.

Any help is appreciated.
 
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#4 ·
You should be able to get codes checked free at places like Autozone and Advance Auto. The parts stores down here all do it free so they can recommend what parts you need to buy from them.

I ended up buying a basic Actron code scanner several years ago and it has been very useful. About $39 back then. I also still have an older OBD I scanner from waaayyy back.
 
#9 ·
The speedometer: bigger tires make the speedo read too low without changing the transfer case drive gear, so that is a probable explanation. Alternatively, if the axles were swapped from, say, 4.10 gears to 3.55 or 3.08 without changing the speedometer drive it would also yield a too low reading. Similarly, if the drive gear was changed to accommodate big 35” tires or a differential gearing change the speedo would read high. While unlikely it ‘could’ be a bad speedo head.
 
#13 ·
Also check every single vacuum hose at each end. Including any hoses going to the heater/ac controls in the cockpit. I would suspect cracked hoses slightly before an intake gasket leak.
 
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#14 ·
trying to think what would be affected by a drop in vacuum and create a fault causing the cel to come on. Timing advance maybe? Only other thing I can think of is a lean condition with an O2 code but that would usually be stored.

is this OBD2 ? forget the year change
 
#16 ·
That is a pretty good resurrection and I didn’t notice- nobody ever responded.

However if your gas mileage is good and just the light comes under load then it could still be a (less severe) vacuum leak, or bad vac to the map sensor, or the IAC. It takes a lot of vacuum loss to even trigger the light non-persistent but to not store a code is unexplainable.

If it’s not storing a code... if you moderately accelerate in say, 2nd gear up to like 4500rpm does the light go out before you let off? If you drive steady after same exercise with the CEL coming on does it go out?

It’s easy to confuse a vacuum issue with a throttle/acceleration issue.

Spark is ecu controlled so not distributor advance.

What is your idle oil pressure and acceleration oil pressure?
 
#18 ·
So CEL is out throttle closed (idle) and goes out at high CFM (WOT or reduced throttle plate opening) which are both higher vacuum conditions.

If you have not replaced your MAP hose and/or sensor that could be a culprit but I’d think it would save that code....

I’m still saying vacuum leak or O2 issue.

Will the CEL come on if you floor it while in 5th? Will it go out if maintaining speed or decelerating in, say, 3rd?

(QUESTION for the learned: does the ecu lock out the up shift light by using the speed sensor input to detect/suspect 5th gear? It so much sounds like the shift light but he says that works separately. ??? )
 
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