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loss of power when hot

17K views 21 replies 14 participants last post by  CoshoctonRuss 
#1 ·
92 yj was running fine until yesterday. while city driving sudden power loss occured. Engine began vibrating as felt through the steering wheel and seat. pressing the accelator pedal felt as if it was doing nothing. I limped off the road and parked it for about 20 minutes. Then started it and slowly drove home (about 4 or five blocks) with little acceleration.

After arriving home, I took it back out on the street and tested it. the power loss then again occurred and I limped home and parked it. Also the oill pressure guage was pegged at 80+ once coming off idle. At idle oil pressure read mid guage.

This morning it started and seemed to run well, oil pressure was fine during all rpm ranges, the engine was smooth and power seemed to be proper. As the engine warmed up the oil pressure would periodically read high. Sometimes with the slightest rpm increase while settling down at higher rpms. The guage performance was erratic. When it became completely warm (temp mid guage -around 210) the power loss returned - first with vibration and shuddering - and no response from throttle- even though I could dlowly get rpms up somewhat but not smoothly, when releasing accelator th yj would try to stall (sometimes would).

Other factors - No CEL light, running codes showed 12 and 33 (normal I think)

This morning test took place with CCV open and line from air filter disconnected (c cv line back to intake capped). The trans was in neutral (did not drive it)

The power loss came on quickly once the engine warmed up. As it began the exhaust pressure appeared strong (while it would still idle)

The problem seemes to be directly related to engine temperature as does the high oil pressure reading.

All suggestrions appreciated

Thanks
 
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#2 ·
I think after a certain amount of time, and a pre-set engine temp is obtained by the computer, it switches from an open loop to a closed loop. closed loop is when the ecm/ecu utilizes the sensors to run the engine. I realize you checked for codes, but it sure seems like a sensor issue.
 
#3 ·
are you sure it is not overheating? your temp guage could be wrong -- it just sounds typical of an overheating engine to lose that much power and run really rough like yours is.
 
#4 ·
paintguy said:
are you sure it is not overheating? your temp guage could be wrong -- it just sounds typical of an overheating engine to lose that much power and run really rough like yours is.
:agree: That is the first thing I thought of also while reading the symptoms
 
#5 ·
paintguy said:
are you sure it is not overheating? your temp guage could be wrong -- it just sounds typical of an overheating engine to lose that much power and run really rough like yours is.
i would think if it was running that hot you would have some other symptoms such as boiling coolant bubbling from the radiator into the overflow tank.....may even see some steam excaping from the hoses or radiator....
 
#7 ·
It does not appear to be overheating. the radiator is full and there is no discharge of coolant. there is no feeling of overheating in the engine compartment. I would lean to some kind of sensor or heat dependent switch that has failed. I am just not sure where to look.
 
#11 ·
Have you inspected the dist. cap and rotor? Whens the last time you changed the plugs and wires? If you haven't changed these in a while, atleast do it to eliminate them. Also, how old is the O2 sensor? I would do the "start simple" and then progress to the more involved/complicated. Keep us posted on your progress.
 
#12 ·
I had an old Landcruiser with these symtoms. Turned out that the distributor shaft bushing was worn and when the engine heated up it was just enough to misalign some distributor parts and produce a poor spark. Check for a small amount of oil where your distributor mounts.
 
#13 ·
i had the exact same problem today at noon, oil pressure spiked to 80 weird steering feeling, felt it in the seat, power loss.... A/c was on, but low. I have an 01 XJ (black)with 50k, engine running at 208F. I dunno about overheating with the temp being what is usually is, no lights came on, i jjust pu8lled into a parking lot, waited 5 minutes, and everything went to normal. Today is supposed to be teh hottest day of the year so says the weather guy, but I don't think it would overheat with the 208 temp. Oh, and i don't have a distributor, COI.
 
#17 ·
This is my first chance to check replies since last post. I am leaning to suspect the temp sensor.
My Haynes manual states " As the coolant temperature changes, the resistance of the sensor changes, providing a varying input voltage to the computer. The computer uses information from the CTS to adjust the fuel/air mixture, ignition timing and EGR valve operation"

My thoughts are that as soon as the coolant comes to operating tempurature, the sensor sends an errored voltage to the computer causing an extreme change to the "fuel/air mixture, ignition timing and EGR valve operation" causing the poor performance.

The Haynes manual continues with the words I hate most " Checking the CTS is beyond the scope of the home mechanic"
How do they know- It may be, and probably is, entirely within the scope if they would publish resistor values for cold and hot running conditions it should be failly simple to check the CTS with a common VOM.

So, in order to confirm or eliminate the CTS, I will remove it, leaving the wires connected and plug the opening with a pipe plug (It appears to be 1/2" IPT).

Since the problem appears when the engine warms up, If CTS never gets warm, the problem should not occur. If it does then something other than the CTS is at fault.

I will post results.
 
#18 ·
93 Sahara with similar issues

Hi,
I get a loss of power right from the get go. I start up the jeep and it sounds to me like the exhuast has a higher pitch (almost a whistle) I have power between 1 and 2000 RPM but anythign after that and the engine is severly underpowered. The odd thing is occaisionally they engine will be fine and rev freely with lots of power (usually happens when shifting to a lower gear, maybe 1 out of 50 times)

The engine heats up a bit but I think it is due to this problem since it will be this way right from a cold start.

I replaced the aircleaner and still the same problem. Any ideas would be apreciated
1993 Jeep Sahara 4.0 manual trans.

Thanks

G
 
#20 ·
ninety 2 yj said:
...The Haynes manual continues with the words I hate most " Checking the CTS is beyond the scope of the home mechanic"
How do they know- It may be, and probably is, entirely within the scope if they would publish resistor values for cold and hot running conditions it should be failly simple to check the CTS with a common VOM...
Here's a temp guage test out of the shop manual. It's mainly for the temp guage but it also tests sender function.

***
1) Turn ignition switch to RUN.
2) Disconnect Coolant Temperature Sender Connector. Needle goes to C. If not, go to step 3.
3) Touch Coolant Temperature Sender connector to ground. Needle goes to H. If OK, replace sender. If not, repair open to guage.
***
 
#21 ·
Loss of Power Too!! (Coil) ???

I also have the same problem with my 95 Rio grande(4cyl). When the damn jeep warms up it sputters/rpm's stay low/backfires sometimes/ just basicly runs like crap! but here is the tricky thing....... last month the previous owner replaced the cap/rotor/plugs/wires/air filter/fuel filter. (I think he was trying to fix this same problem) but on a 15 min test drive it didnt do it(damn him) So i started where he left off.... THE COIL! i bought a new one for 30 bucks and guess what it worked but not for long after that one warmed up it did the same thing again. now every time the jeep starts to sputter i pull over and swap coils(30 seconds) and then it runs great again until it warms up again? what can cause this? I swap between my old coil and my new coil all the time. anything over 2000 rpm is useless!

Keep in mind that I dont even give my engine a chance to cool down between swaps. it only takes me 30-60 seconds to change over coils and then i start it right back up!

Did any of you find a way to fix this problem with your own Jeep?

Please advise ASAP... Thanks

95 Rio Grande
5-spd
Tan with Spice
157k miles
 
#22 ·
I had this issue (minus the oil pressure spike) in my 4.0 1990 Cherokee and it ended up being that my fuel pump was clogged in the tank. It was making a buzzing noise too, which I could have sworn was coming from the exhaust. I cleaned out the tank and put in a new pump and it fixed it. Hope this helps.
Russell
 
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