im stumped

94 xj limited, OBD1

it has an off idle hesitation, once warmed up its ok, then if i let it sit, its bach again

with this it does this thing where its like raves up 500rpm and then drops, revs up again, if i let it idle its ok, if i rev it over 2000 rpm its ok, once its running its generally ok.

I've changed the coil, spark plugs, changed the TPS twice, checked for a earth to the TPS checked the 5 volts, checked for just under 1 volt at idle and checked for a smooth increase in voltage with an analogue meter, checked resistance also, and it seems ok

I've replaced the IAC and that made no difference,

I've checked for vacuum leaks, and there doesn't seem to be any,

the crank angle sensor has been replaced a few years ago.

I'm at a loss as to know where to go next,

any ideas???

noon raves, REVS

gees i getting worse, where the edit key

lets try again " NO not RAVES, REVS" stupid key board

Does the 500rpm jump correspond to the A/C kicking in (hvac on defrost)?

definitely not a/c, its as if the ignition was being rapidly turned off, on,off,on.

ive even noticed on a slight down hill run foot off the gas that the tacho would oscillate up and down about 500 rpm.

You may have a fuel injector that is leaking fuel and not shutting off. Every time it leaks the RPM goes high for a moment until the excess fuel is burned/purged/exhausted and then the RPM drops and repeats the high-low RPM cycle. Either remove each injector and clean/check or run a couple of injector cleanings to see if that reduces the RPM cycling.

Best regards,

CJR

that's an interesting idea, I've been thinking go an injector, especially was they are original and have done 440,000 kilometres, ill try injector cleaner first, and see what happens.

i may need to think of doing an injector upgrade as preventative maintainance.

CJR said:

You may have a fuel injector that is leaking fuel and not shutting off. Every time it leaks the RPM goes high for a moment until the excess fuel is burned/purged/exhausted and then the RPM drops and repeats the high-low RPM cycle. Either remove each injector and clean/check or run a couple of injector cleanings to see if that reduces the RPM cycling.

Best regards,

CJR

Click to expand...

If you're going to the trouble of pulling them, it might be worth just replacing them. $100 will get you a set of refurbished Bosch.


I got a set of refurbished Bosch 746 injectors for my 89 from jhc7399 over on cherokeeform.com for $70 (my originals started weeping externally). I think you need the 703 for a 1994, which I think he was charging $100 for a set. Don't buy the cheap knock offs from eBay.

thanks, I've done a bit more of an assessment now and wonder what you make of this, I'm feeling confident that it is the injectors.

i thought at first it might have been heat soak, as it would be ok when cold, and only play up after sitting for a while after driving, once running it stays ok.

so i filled up with 98RON fuel and added a bottle of injector cleaner, and after running, it has started to run about 50% better. ive also noticed that when it plays up , if i rev it a couple of times it stabilises.

so my thoughts now are that i have at least one leaking injector if not more, and that obviously when pressurised, as they would be just after use, that would be when they leak.

and to clear an over fuel problem reving it would clear up the excess fuel.

so now i need injectors, i thinking i may as well up grade to 4 hole and probably BOSCH 0280150965 i believe i can use these without having to change my fuel pressure regulator

i gave a '94 OBD1 cherokee 4.0l

any suggestions

Ewemn,

In my view, as long as the lbs/min or lbs/hr of fuel rate is the same, for the one(1) hole injector and four(4) injector, your regulator should be fine. Typically, the cross-sectional flow area of one(1) injector hole is equal to all the four(4) hole flow areas of a four(4) hole injector. The benefit, of multiple hole injectors, is that the atomization of the fuel is improved over a single hole injector. 100% atomized fuel spray, rather than a fuel spray containing partial atomized fuel + fuel droplets, provides better combustion for more power and better mileage.

Looks like you're getting close to solving your problem.

Best regards,

CJR

ok this is doing my head in, i got the bosch 4 hole injectors and they work beautifully, as has been quoted, smoother idle slightly healthier acceleration, and maybe marginally better economy, that's when i don't use the healthier acceleration heheh.

however the problem is still NOT fixed, so new theory.

so now I'm suspecting the MAP sensor, and the theory is,

when i start the car it idles smooth enough but if i quickly blip the throttle the engine stumbles, however once up and running its fine, unless i stop and park it for a short time 15 minutes will do it.

so anyway i have it in the shed and its playing up, so i remove the vac hose from the MAP sensor and plug it, the engine runs, but the tone changes and the off idle flat spot is gone, re connect the vac line and the stumble returns, go through the process on different days and same result.

so ive ordered a replacement from rock auto, now just got to wait 10 days for it to arrive
(Australia)

hopefully this is THE theory that works.

time will tell, sad thing when its running its as sweet as.

oh and the original injectors came out perfectly clean, 25 years old and 440000 km
not bad hey, i use 98 octane 4 or 5 time a year i guess it works keeping these things clean.

thanks for reading, wish me luck heheh

yea, FIXED woo hoo

it was the MAP sensor.

this old girl aint ready to die yet

so happy, and upside is all the extra stuff i did is making it run beautifully

so filled the tank with fuel and interested to see if there are any economy improvements.

you have no idea how relieved i am.