Topic: YJ idles ok but lacks power while driving

[Eucalypta]

Hello Fellow Jeepers,

Thank you for joining your forum. I immediately have a question, have searched the forum and it somewhat seems like the problem sirshessire has.

When my 1994 YJ (MPI) is cranked, it starts immediately, when idling, no problem, more revs, no problem.
Driving it cold, still no problem but just after 2 or 3 miles the troubles start; when accelerating or just shifting to a higher gear and keeping the speed the same, the engine starts to hickup, it looses power and I am forced to release the gas. When accelerating again, same thing happens and I have to slow down even more until I come to a halt where the engine purrs like a kitten and Idles great.
When not under a load the engine revs ok.

Fault codes: 21, 27 (O2 sensor and injector) 12 33 55 (OK)

Things that happened in the 500 km before this happened:
-Had some water in the fueltank; tank emptied, tank cleaned, installed new fuel filter, injector rail cleaned, injectors cleaned, fuel lines cleaned. After this the car ran great again.
-Had the YJ 2,5" lifted by installing a spring lift
-Renewed my engine oil
-reattached the Fuel filler vent hose
--> Problem occured for first time after driving 100 miles
-noticed 2 vacuumlines detached: valvecover vent(from vacuumbody to valve cover), vacuum to PCM. Both reattached, no sollution

Things I checked:
-Fuel pressure under vacuum 31 PSI (OK) without vacuum 39 PSI(OK)
-unplugged  the injectors one by one, all injectors work (while idling)
-unplugged the TPS Idle went bad, when connecting again Idling went great again
-All vacuumlines are attached and ok, all vaccum lines operate under vaccum (tested)
-reset computer, after this no fault codes are shown (only 12 33 55)

Has anyone an idea what the problem might be?

[Jeffy]

Well, since you're getting an O2 code then I suggest replacing that.  If the CEL comes on with a O2 code you'll notice the stumble in the engine.  It's like you're running on 3.5 cylinders.  When mine went back, it was OK when idling and driving around town.  When I took it on the highway where you're holding the throttle and RPM's at a set number it would act up.  At first it wasn't as predictable but later when it got worse it was like clockwork.

[sharpxmen]

other than O2,
make sure you have no check valve on the vacuum line going to the MAP sensor (EDIT: should be a plain vac hard plastic line going from the intake manifold to the MAP sensor vac port - it should not be a rubber hose either, some tend to collapse under vacuum), make sure the catalytic converter flows well and also test the tps output. if all this turns out ok i would maybe have a look at the spark plugs, wires, dist cap and rotor, but since you say it's fine unless you are moving it's less likely - check the look of the plugs anyway to make sure (color and gap)

i'm inclined to think that is the cat, it can stumble pretty bad if it's plugged or broken pieces inside are plugging the output (EDIT: it would idle just fine). keep us posted

[Eucalypta]

Thnx, Just home from some wheeling with the jeepclub (was a passenger this time); will check these things tomorrow.
The cat doesnt sound plausible; it runs fine when engine is cold; as soon as it ran just 1 or 2 miles (temprise just showing) the problem occurs.
When does the motor management kick in? right away or when the engine is on operating temperature?

[sharpxmen]

Thnx, Just home from some wheeling with the jeepclub (was a passenger this time); will check these things tomorrow.
The cat doesnt sound plausible; it runs fine when engine is cold; as soon as it ran just 1 or 2 miles (temprise just showing) the problem occurs.
When does the motor management kick in? right away or when the engine is on operating temperature?

closed loop occurs once the O2 sensor is up to temperature - something like 2 to 4 minutes i think. when is the last time you changed the cat?

[johnbrakingground]

This sounds like it needs a new fuel filter.  The engine requires more fuel under a load than at idle. I have had several cars do this same thing in the past.  Your getting the 02 sensor because it is running lean. That also explains the bucking its starts to starve for fuel so the engine almost dies which gives just enought time for the pump to fill the lines with fuel again and the process repeats itself.    When you first start your Jeep the fuel has filled the whole system so it takes a little time to start starving itself.  Please get back after the fuel filter replacment just to see if I was right.  Good luck mang

[johnbrakingground]

After reading your post again I saw that you replaced the filter.  Cats plugged?

[Eucalypta]

The cat seems open; at least enough to pass the fumes.
Exhaust gasses are clear; no black/blue or white smoke; seems the mixture is too lean.
Makes sense that the fuel line/filter/pump is defect however I do get enough pressure; still have to check the flow.
I am now thinking of a mallfunction in the throttlebody, maybe the TPS is the gremlin after all....
Will exchange the TPS with the one from my Grand Cherokee; they have the same partnumber.
Ill let you know if this helped.

[sharpxmen]

The cat seems open; at least enough to pass the fumes.
Exhaust gasses are clear; no black/blue or white smoke; seems the mixture is too lean.
Makes sense that the fuel line/filter/pump is defect however I do get enough pressure; still have to check the flow.
I am now thinking of a mallfunction in the throttlebody, maybe the TPS is the gremlin after all....
Will exchange the TPS with the one from my Grand Cherokee; they have the same partnumber.
Ill let you know if this helped.

if you have pressure (under load) you have flow (if you don't the pressure drops)

[jfrabat]

I agree that it COULD be a cat or an O2 sensor; but I would not rule out lack of fuel.  My '94 had similar simptoms, and it turned out that the fuel pump was dying on me...  Not completely dead, but could not maintain pressure while under load. 

[sharpxmen]

I agree that it COULD be a cat or an O2 sensor; but I would not rule out lack of fuel.  My '94 had similar simptoms, and it turned out that the fuel pump was dying on me...  Not completely dead, but could not maintain pressure while under load.  

yeah, could be, he should monitor the fuel pressure while driving - if it drops then it's the fuel pump/filter or the FPR (although in that case i think would have a pressure issue when not driving as well).

EDIT: i would check the cat though, imo all the symptoms point towards that including the fact that it runs fine when cold so would be interesting to know when was the last time he replaced it, they don't last forever even if they're stainless, the insides get clogged or broken and the exhaust gets plugged - one thing to listen for is a hissing noise, the exhaust gases would find their way out at the joints. He also mentioned in another thread that  there's exhaust smell under the hood.

[aw12345]

What was the thing that Chris went through, with the tank hitting one of the tubes in the tank? Didn't he have hard, start, lack of power and several other similar problems?

[sharpxmen]

What was the thing that Chris went through, with the tank hitting one of the tubes in the tank? Didn't he have hard, start, lack of power and several other similar problems?

i think he hit the tank skid and the bottom deformed pressing the plastic against the pump inlet and causing lack of fuel flow under load. but in Chris' case i think he had issue starting it as well (if i remember correctly), but yeah, this is another thing to check (all relates to fuel pressure under load though so should be visible on the gauge while driving).

[jfrabat]

I agree with sharp here; put the pressure gauge on the Jeep, and take a drive around town; if the pressure drops significantly, you just narrowed your probles to fuel (so it can be either the filter or the pump).  Replace the filter first and then the pump if this is the issue.

[aw12345]

I always like reading code descriptions, so spend some time perusing the YJ repair manual, for starters wide open throttle the YJ will run in open loop so if falls on it's face then it's some sort of fuel delivery  problem and the the engine doesn't give a hoot what the oxygen sensor tells the pcm it is ignored and just gets the maximum amount of fuel that the fuel table in the pcm allows for the current situation, depends on coolant temp and map sensor value as well as engine rpm those are the main inputs for wide open throttle operation.
The injector code is kinda funny if it comes back that bears deeper investigation.

27* . . . . . . . . . . Injector #1 Control Circuit Injector #1 output driver does not respond properly to the control signal.
or
Injector #2 Control Circuit Injector #2 output driver does not respond properly to the control signal.
or
Injector #3 Control Circuit Injector #3 output driver does not respond properly to the control signal.
or
Injector #4 Control Circuit Injector #4 output driver does not respond properly to the control signal.

The oxygen sensor code has 2 descriptions that are kind of different

21** . . . . . . . . . . O2S Stays at Center Neither rich or lean condition detected from the oxygen sensor input.
or
O2S Shorted to Voltage Oxygen sensor input voltage maintained above the normal operating range.

Get the engine hot and tab into the oxygen sensor wire with a digital volt meter or use a scanner with a chrysler cartridge and check the oxygen sensor voltage it would not surprise me at all if it toggles just fine. Then try to drive it rich and lean for this a scanner is really handy you basically touch the sensor wire on the side that goes to the pcm with your finger then with a finger on the other hand touch either the positive or negative terminal, this should drive the voltage reading of the oxygen sensor on your scanner high or low. 0.8 volts or around 0 volts. If it does all that, you simply have some sort of fuel delivery problem, have you tried taking the vent hose of the filler neck back off? From the description of your symptoms, my money is on some sort of tank venting problem. have dealt with very similar problem in the   past on cars and truck, carburated or fuel injected, even diesels act must the same when they starve for fuel. When it starves for fuel the oxygen sensor will bias lean and stop toggling hence the oxygen sensor code