Why would a jeep rev up and down with constant acceleration held,

ste2006

Hi All,

I have a 91 SWB Suzuki SJ413 (Small Jeep), It is Throttle Body Injection but i am having an issue with the engine,

When the engine is cold its fine but once it heats up it revs up and down on its own at idle but even when i hold a constant acceleration it revs up and down by anything up to 1000 RPM,

I have put loads of work into the engine and it was ok but now this is happening,

It got so bad once i was driving down a passage and the jeep was literally jerking forward because the RPM kept going up and down,

Anyone any thoughts,

Thanks,

Stephen

Eric Cartman

1000rpm sounds a bit much but heres 2 posibilities , is it automatic ? does it have air conditioning ?

ste2006

No its manual and no airCon,

This is fairly old, I read somewhere that it may be an air leak or vacumn leak, Any thoughts????

MetzgerMeister

Could be a faulty MAF (mass airflow sensor) or o2 (lambda) sensor.

ste2006

Any ideas how to test these???

It seems to be a bit better now again, Also it only happens once the engine heats up.

If it helps it only started happening after the jeep got a hard bang (Just bounced down a very very steep slope and accross some bog

MetzgerMeister

ste2006 wrote: »

Any ideas how to test these???

It seems to be a bit better now again, Also it only happens once the engine heats up.

If it helps it only started happening after the jeep got a hard bang (Just bounced down a very very steep slope and accross some bog

Take out the MAF and give it a clean. If it's the o2 sensor, it's a heated one. This would be why it happens when the engine heats up.

gleep

AFIK, a slipping clutch will cause the revs to shoot up and down, no expert but I've had this problem.

ste2006

einheisserschre wrote: »

Take out the MAF and give it a clean. If it's the o2 sensor, it's a heated one. This would be why it happens when the engine heats up.

Ill try clean the MAF, Any ideas for cleaning it???

The engine is quite old and basic, would it still have an O2 sensor that controls the air/fuel ye????

Assuming the O2 sensor is just for emmisions is there any way to loop it out as this jeep is a long way from road worthy and definitely wont be going for its NCT anytime soon

ste2006

gleep wrote: »

AFIK, a slipping clutch will cause the revs to shoot up and down, no expert but I've had this problem.

Clutch is a brand new high performance clutch and it also happens when idling so i dont think its the clutch

AzureAuto

The huntimg sympton you are describing is typical of a faulty air flow meter, or a vacuum leak upstream of the throttle butterfly. the response by the engine management to excessive air in the emissions as detected by the O2 sensor would be to balance it out with an increase in fuel injection. However, if the air flow meter detects less air, either because of a fult or a leak, then the original calculation for fuel injector pulse width would be incorrect. hi occurs cyclically hence the rise and fall in engine revs. Have a good root arond for split vacuum pipes and hoses, also give throttle body a good clean out with a carb spray and check for any breather hoses that may be gunked up.
To test the air flow meter you would need to know operating voltages. Usually there is a 5V feed going to the sensor, earthed by the engine managment ECU. With ignition set to position 2 check the terminals of the connector (disconnected) to the sensor for this. Then with tengine running and an assistant at the throttle pedal, backprobe the connector terminals and check for a steadily/ smoothly decreasing voltage between the signal terminal and earth of the air flow sensor connector as the assistant presses the pedal to the floor. This is a basic test. An oscillospcope or diagnostic computer would be better suited, but Im assuming you have neither of these.
Good luck.

ste2006

AzureAuto wrote: »

The huntimg sympton you are describing is typical of a faulty air flow meter, or a vacuum leak upstream of the throttle butterfly. the response by the engine management to excessive air in the emissions as detected by the O2 sensor would be to balance it out with an increase in fuel injection. However, if the air flow meter detects less air, either because of a fult or a leak, then the original calculation for fuel injector pulse width would be incorrect. hi occurs cyclically hence the rise and fall in engine revs. Have a good root arond for split vacuum pipes and hoses, also give throttle body a good clean out with a carb spray and check for any breather hoses that may be gunked up.
To test the air flow meter you would need to know operating voltages. Usually there is a 5V feed going to the sensor, earthed by the engine managment ECU. With ignition set to position 2 check the terminals of the connector (disconnected) to the sensor for this. Then with tengine running and an assistant at the throttle pedal, backprobe the connector terminals and check for a steadily/ smoothly decreasing voltage between the signal terminal and earth of the air flow sensor connector as the assistant presses the pedal to the floor. This is a basic test. An oscillospcope or diagnostic computer would be better suited, but Im assuming you have neither of these.
Good luck.

Wow,

Thanks for this, I may be able to get my hands on a scope ( i have a nice small one somewhere)

Ill try this, If it makes any odds i have the breather hose from the rocker cover disconnected and just pointing to the ground as i replaced the air filter with a performance one.

I will try to test or replace the MAF and see if this helps the problem,

I also have an issue when the jeep is not level it is much worse, if its dead level the effect is not as bad, Would this point to anything???

Ill try the ideas above and replace the MAF (I think i have a spare one) and see if i can get it going a little better.

MetzgerMeister

ste2006 wrote: »

Ill try clean the MAF, Any ideas for cleaning it???

The engine is quite old and basic, would it still have an O2 sensor that controls the air/fuel ye????

Assuming the O2 sensor is just for emmisions is there any way to loop it out as this jeep is a long way from road worthy and definitely wont be going for its NCT anytime soon

Just use an alcohol prep-pad to clean the element.

If your jeep has fuel injection then it will have o2 sensors. They help determine how much fuel:air is needed and send that info back to the ECU.

LordSutch

I take it that the word 'Jeep' now covers all 4X4s (Suzuki, Land Rover, BMW, Nissan, Opal, etc) ? :cool:

ste2006

Camelot wrote: »

I take it that the word 'Jeep' now covers all 4X4s (Suzuki, Land Rover, BMW, Nissan, Opal, etc) ? :cool:

what would you like me to call them????

Suzuki 4WD ATV

SouperComputer

Id advise against touching the MAF film in any way, including a prep pad.

With the ignition off, disconnect the MAF then start it up. The ECU will fall back to "failsafe" mapping when is no longer sees the MAF. Its a crude but often effecive way to test the MAF. If the idle seems better then its likely the MAF. Often this map is on the rich side to say the least, so dont run it with the MAF disconnected for longer than you need to.

As also pointed out, check for vacuum/intake leaks. Check all hoses, connectors and junctions.

Has the check engine light come on?

ste2006

SouperComputer wrote: »

Id advise against touching the MAF film in any way, including a prep pad.

With the ignition off, disconnect the MAF then start it up. The ECU will fall back to "failsafe" mapping when is no longer sees the MAF. Its a crude but often effecive way to test the MAF. If the idle seems better then its likely the MAF. Often this map is on the rich side to say the least, so dont run it with the MAF disconnected for longer than you need to.

As also pointed out, check for vacuum/intake leaks. Check all hoses, connectors and junctions.

Has the check engine light come on?

Ok ill try that instead so,

No Engine light has not come on athough i dont even know if it has one, Its that old

tc2010

I had the exact same problem and it turned out to be the throttle body. A quick clean and it was sorted

ste2006

tc2010 wrote: »

I had the exact same problem and it turned out to be the throttle body. A quick clean and it was sorted

Is it just Carb cleaner you used to clean it???

tc2010

i got it done in a garage. probably half an hours labour. I preffered to get it done by someone with experience as other parts had to be taken out to get at the throttle

my car was just akward. most cars are easy to get at the throttle

People generally clean it with carb cleaner. Plenty of videos on youtube and that showing how its done

ste2006

Great thanks for that, Ill try it over the weekend and let you know how i get on,

Thanks,

Stephen

Pkiernan

Since the car is nealy 20 years old, I'd be taking a good look at all the hoses and vacuum lines.

A leak in one of these will cause the problem you describe.

You can also spray carb cleaner on the lines while the problem is occurring. It it stops for a while when you spray the carb cleaner, you have found the leaky hose.

Detailed instructions here:

http://www.ehow.com/how_5584244_auto-vacuum-leak.html