88 Sahara I6 - Odd RPM Issue

Alphasig83

Alphasig83

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Joined
Feb 5, 2022
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Bucktown PA
Hey all, I have an odd problem that has me scratching my head... I have a stock 1988 6cyl. Carbureted Wrangler with 10,000 original miles on it. Original carburetor. Most items on the unit are factory original for context... I'm also keeping this unit 100% stock as it is such a cool timepiece.

The other day, I put a fuel cleaner in the gas tank when I filled up, and about a day after, I started to get a problem where the Jeep's RPM seemed like they were bogging down, and it got worse to the point where I would floor the pedal but only be able to get to about 3000 RPMs, and then the rig would stall out. Of course I first thought about the carb... I took off the carb and cleaned and rebuilt it (twice). I then bought a new carb of the same model (Carter) and put it on, and had the same issue. I have good, smooth idle at about 7-800 RPMs, responsive acceleration up to about 3000 RPMs, but nothing else.

I also thought this could be a transmission issue, but when I have the Jeep in park and attempt to rev the engine, I can only get to about 3k...

I have cleaned the fuel filter, confirmed the fuel pressure is good, and the fuel lines are clear. I'm thinking this could be some sensor issue... any thoughts??

Thanks in advance!
 
Alphasig83 said:
Hey all, I have an odd problem that has me scratching my head... I have a stock 1988 6cyl. Carbureted Wrangler with 10,000 original miles on it. Original carburetor. Most items on the unit are factory original for context... I'm also keeping this unit 100% stock as it is such a cool timepiece.

The other day, I put a fuel cleaner in the gas tank when I filled up, and about a day after, I started to get a problem where the Jeep's RPM seemed like they were bogging down, and it got worse to the point where I would floor the pedal but only be able to get to about 3000 RPMs, and then the rig would stall out. Of course I first thought about the carb... I took off the carb and cleaned and rebuilt it (twice). I then bought a new carb of the same model (Carter) and put it on, and had the same issue. I have good, smooth idle at about 7-800 RPMs, responsive acceleration up to about 3000 RPMs, but nothing else.

I also thought this could be a transmission issue, but when I have the Jeep in park and attempt to rev the engine, I can only get to about 3k...

I have cleaned the fuel filter, confirmed the fuel pressure is good, and the fuel lines are clear. I'm thinking this could be some sensor issue... any thoughts??

Thanks in advance!
Click to expand...
Hey, if anyone follows this in the future... I'm trying to disconnect my catalytic converter... I've watched a few videos where the cat fails and plugs the exhaust a higher RPMs... we'll see...
 
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USA spec car right>? USA, 4.2L engine,

bad fuel, 88 ',4.2L ,carb YJ,
is that a huge typo , 10k miles? or 100k (not nagging) but matters.
23 miles driven per month or 1 mile a day (less)?????? really?
that would be bad fuel. and all the problems with that.
100% sure the heated choke is 100 % open hot engine? must be. or wired wrong it is. the oil pressure switch drives the choke stove. over 4psi.
the carb has a MCU computer, complex. it must be scanned to find out what is up with it. and has 1 O2 sensor too.
unplug the Knock sensor to see if spark timing greatly improves. (fSM even states that)
the MCU does connect to the dizzy. so there are 3 kinds of spark advance, and MCU knock , will retard it some.

if the VACUUM tool test look good the CAT iS not melted. ( see vacuum go backwards gunning the throttle, check the cat.)
19" Hg.idle, when gunned if it it drops hard good, but if rises HG. backwards , bad cat. (melted) cheap as dirt tool. as many are today.


OK it fails at idle ,parked, so is 100x more easy to fix than any other moving only problems (driveability)
PARK is A/T so yes a TC hard failure is in the list.
but vastly more easy tests prove it is not. for sure bad fuel or spark bad.
some one played with the dizzy or installed it wrong,"? now grossly retarded.?

First is use a spark timing light, they are dirt cheap, now.
connect it and see if spark goes dead at or after 3k rpm.
makes sure spec advance happens. of damper and timing strobe lamp pointed to scale there.

in park and try too ,neutral, w/ chocked tires.

make sure A/t box ATF lube is above the line and never overfilled ever. do it by the book and is never checked like engine, dipstick way)
see if spark fails, first, so easy
fuel filters packed up, not one lasted 35years. aircleane, spark parts good?,, rotor/cap/hv-wiresets and plugs?)and gapped to spec not out of the box 0.045 ever) never. 0.035" spec.

also see if spark advance is dead. per the FSM , got one yet they are free. and linked here this site
stock is great, !!! everyone can fix a stock car, but with the mods and hacks , not easy!, some are a horror hacked, for on road trips, OMG what is this stuff.

you buy this new? 35 years ago, if not look for NAnny/TEEN HACKS, rpm, LIMITERS (governors), LOOK AT ALL EFI WIRING FOR SIGNS HACKING.
Gov connected to DIZZY. hacked in?
nobody puts GOV on CARB that be crazy hard.
bad ICM

there is 1 computer here MCU, the old dizzy here 4.2 has both vacuum advance and mech, centrifugal. both
chapter 8D,page 33 covers the tests, using timing light, if failing this full power will be pathetic, just like silly lawn mower lacking true spark controls are.

the centrifugal advance is 12deg at 2000 rpm. if the guts inside, Centrifugal are rusted as most this old are, then there is no advance but vacuum type.
the vacuum tables same page show up to 27 degrees advance. 8D-38

if the tranny was bad you'd know that, not just limited RPM , for sure.

bad fuel /bad spark ,if compression is good. (and cat not melted sure)
one more causes EGR leaking stuck open will do it. open too far.

the FSM manual is key here, for sure MCU controls, to all . (spark and fueling is MCU controlled)







the advance charts good luck figuring out any old car like this with MCU controlled carb and distributor.

adv12.JPG
 
the engine is an air pump,( base)
if it can't blow air to the CAT, then you measure huge over 2psi at the 02 flange bung hole 02 pulled! test 1:
and if there is huge pressure there cat melted then test 2 fails a cheap vacuum gauge.

the engine cant even pump air, and so can not suck a vacuum on the intake side too,.
vacuum tests are so simple here, no need to spend all day dropping header bolts rusted to death.
what you would see is idle vacuum weak, then gunning the throttle vacuum goes tally crazy, first do vaccum\ tests in good engine
then do this one, see how VAC tests fail
 
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