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Erratic throttle response and uneven idle

 
 
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andyff
Just got MTs


Joined: 18 Jan 2010
Odometer: 141
Location: Wakefield



PostPosted: Fri May 08, 2015 12:21 pm    Post subject: Erratic throttle response and uneven idle Reply with quote

Hi.

My newly acquired 1997 4L Cherokee has developed a fault. At idle it can sometimes become erratic climbing 500 rpm over normal then dipping 200 below. Not all the time however.

Combined with this and more noticeable around town, from time to time it feels as if the power has gone, but if I push the pedal further it will pick back up and deliver plenty of power to full revs. There is no lack of power if I floor the throttle, it is with constant pedal position it is just as if I have lifted off the pedal slightly.

Plenty of advice on US forums suggesting fuel pump, but here is the clincher. This Jeep has an LPG conversion and does this same thing on both petrol and gas.

Any thoughts or ideally anybody had the same problem?
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jeremy
Just got MTs


Joined: 09 Jan 2006
Odometer: 467
Location: Lydd Kentshire



PostPosted: Fri May 08, 2015 1:31 pm    Post subject: Reply with quote

If it has one, I would say the throttle body needs cleaning or replacing if the potentiometer or what ever it is that controls the throttle position is cream crackered.
__________________________________
Nissan patrol, Jimny.
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andyff
Just got MTs


Joined: 18 Jan 2010
Odometer: 141
Location: Wakefield



PostPosted: Fri May 08, 2015 5:46 pm    Post subject: Reply with quote

Hi,

Thanks, I will investigate that tomorrow.

I have just made some more observations.

Oddly when the revs fall at idle the electric fan starts, might even be the fan starts marginally before the revs fall. Only runs a few seconds and the revs lift a bit and fan stops again. Temp gauge works and reads a solid 100 all day long. When I was testing in the garage it only got to 40 given it was only idling (or trying) for 5 minutes. What else other that temp tells the electric fan to run?

I was also aware of a hiss that lasted briefly after the engine was switched off. Could be a vacuum issue. There is an odd plastic cylinder mounted on the bulkhead connected to the inlet manifold by a small rubber tube and with another disappearing off goodness knows where. It feels to have something "bouncing around" inside. Anybody know what that is?

Thanks in advance
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andyff
Just got MTs


Joined: 18 Jan 2010
Odometer: 141
Location: Wakefield



PostPosted: Wed May 13, 2015 9:28 am    Post subject: Reply with quote

In case anybody is suffering from similar engine symptoms I will mention where I’m at so far in my investigations.

The electric fan running every few seconds as the engine revs dipped was misleading me to assume that the loss of power and running fan were related. They are in fact two different faults.

The AC is supposed to run in 5 out of the 7 position that can be selected for ventilation options on the post 97 face lifted Cherokee. So even in demist with the heater set to hot it runs to dry the air. Whenever the AC switches on, the pressure in the system increases and this triggers a sensor that leads to the electric fan running regardless of engine temp. So whenever the AC is on the electric fan runs. Now if the AC system is low on refrigerant then the AC pressure only momentarily climbs to a level to trigger the sensor that detects the pressure as risen and then drops. The AC is then turned off presumably to avoid damage and the electric fan will also turn off. After a few seconds it tries again and this is why the fan keeps cutting in and out.

I hadn’t noticed the AC compressor clutch cutting in and out, and no doubt the extra load on the engine due to the AC and electric fan turning on was leading an already rough idling engine to drop its revs still further.

Now Jeremy above is almost certainly correct (I hope) about the TPS in the throttle body. I assume this tells the computer the current throttle position so it can compensate by whatever means for additional load. So with the vent position in one of the two modes that won’t switch the AC on, no electric fan running and idle is still rough but not as bad as with these additional loads cutting in and out every few seconds.

So new TPS or throttle body is my next job I guess.
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jeremy
Just got MTs


Joined: 09 Jan 2006
Odometer: 467
Location: Lydd Kentshire



PostPosted: Wed May 13, 2015 9:42 am    Post subject: Reply with quote

Take it off and clean it before buying a replacement, Throttle bodys can get quite gungy inside leading to them sticking at idle. If that don't work it's probably the position electricary bit. Older Volvo's suffered with a similar thing.
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andyff
Just got MTs


Joined: 18 Jan 2010
Odometer: 141
Location: Wakefield



PostPosted: Tue May 26, 2015 9:47 am    Post subject: Reply with quote

Hi,

I thought I would do an update for anybody else who has this issue.

I removed my throttle body and cleaned it, even though it was already cleaner than I had expected. I then fitted a new TPS fresh in from the states. Re assembled and it was the same. Still un even idle when left to idle for a while and still times when with constant throttle the power drops away but comes back with a further pressing of the accelerator.

Another thing I'm aware of now is that the cruise control doesn't work. The light comes on, on the dash but it makes no attempt to hold speed when set. I ran out of time, but I'm thinking of plugging the vacuum pipe to the cruise control unit and seeing if perhaps their is a vacuum leak.

Other than that I'm out of ideas. Can't be fuel related as it does the same on petrol and gas. Maybe CPS?

Cheers guys
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andyff
Just got MTs


Joined: 18 Jan 2010
Odometer: 141
Location: Wakefield



PostPosted: Sun May 31, 2015 9:32 pm    Post subject: Reply with quote

Hi

Another update for a poor unfortunate who might google search this fault in the future.

So no vacuum leek that I could detect.

I decided to strip and re-clean the throttle body and clean the IAC at the same time. After this, no change still terrible idle leading to it stalling and a lack of power whilst driving every so often.

Then I decided for the heck of it to disconnect the two O2 sensors. Well after a few minutes of wandering idle rather than the dramatic racing and dropping that it normally develops, idle evened out.

After 40 minutes idle (never managed more than 4 before) I took it for a drive, both on petrol and gas. No issue, drove fine and idled almost (but not quite) fine.

Reconnected O2 sensors and problem returned.

So, I guess some new O2 sensors should do the trick. Fingers crossed!
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RichardD
Marshall


Joined: 13 Mar 2003
Odometer: 22856
Location: State of Confusion



PostPosted: Mon Jun 01, 2015 7:48 am    Post subject: Reply with quote

It's always good when you find the problem. usually electronics these days!
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Poking the Grim Reaper with a stick then running away. The devil made me do it but God said it was okay with him.
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andyff
Just got MTs


Joined: 18 Jan 2010
Odometer: 141
Location: Wakefield



PostPosted: Sun Jun 14, 2015 4:42 pm    Post subject: Reply with quote

If only I had fixed it Richard.

The new O2 sensor arrived and low and behold the problem is the same although dare I say slightly improved. Still idle gets into this oscillation where the revs rise and fall more and more and still power falls away at constant throttle every so often.

Whilst idling erratically i disconnected the O2 sensor and it immediately started to idle properly again.

So, I think disconnecting the idle sensor sends the ecu into some "safe" open loop mode. With the ecu connected it is in closed loop mode and some part of that feed back system is failing.

But what is it??????????????????????????????
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weasel
Just got MTs


Joined: 29 Dec 2012
Odometer: 167




PostPosted: Sun Jun 14, 2015 6:26 pm    Post subject: Reply with quote

I have never had a car that can drive you made over stupid little problems like Jeeps do with my XJ it is starting one time it will start first time the next time it will crank six times and then start quickly on the second SPAM only Jeep could have not put immobilizer and all of the other functions in with the engine computer we would stand a fighting chance of finding what the problems are.

I hope that you can get your problem sorted in the end I am going to leave it until one day it will not start at all and then we ether find the problem or scrap the bloody thing looking on the bright side if it was a Land Rover I would be expecting it to rot away not a problem with my Jeep.
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