Vacuum Line / Fuel line Issue?

[Chris S]

Jeep for Reference: 97 4.0l 5speed manual

Recently (past few months) my jeep has been doing something quite odd... When starting up the jeep and driving it for the first couple minutes it completely dogs. It acts as if there is not enough fuel for it to get up to speed when in first, second, and third, until I have to basically gun it and almost hit rev limiter just to get up to the speed limit required on the roads nearby. The issue then vanishes after several minutes of driving and its completely fine no matter the gear.

We initially figured it must be the fuel pump or fuel filter. We replaced both and that didnt solve the issue. We took it into a shop and they replaced the distributor cap and did normal maintenance on it. Spark plugs are all good and still the issue. My only guess would be vacuum lines.

The vacuum lines are 20 years old so i would understand them being porous and/or having some sort of issue. Since it seems fuel related I have narrowed it down to the vacuum lines going into the charcoal canister. Starting/running the car with those vacuum lines on or off makes no difference.

Could it be that when I start it up there isnt enough vacuum and its not receiving enough fuel, and its not until the fuel lines are full of fuel that is able to hold substantial vacuum? Leading to me just having to replace the vacuum lines going from the fuel line/fuel tank into the charcoal canister?

Has anyone else had this issue or something like it?

Any ideas?

 

[JP98]

Try this. Cheap and easy. https://wranglertjforum.com/threads/how-to-clean-your-iac-idle-air-controller-valve.662/

 

[Chris S]

Try this. Cheap and easy. https://wranglertjforum.com/threads/how-to-clean-your-iac-idle-air-controller-valve.662/

Thanks, im going to try that as soon as I can. Hopefully it does the trick.

 

[Brantley]

It shouldn't be the IAC, because it's fully bypassed by the butterfly valve when driving around. Although, cleaning it never hurts of course. But, it shouldn't ever make a difference when you're pressing the gas.

Have you checked for any codes? My guess would be a sensor issue of some sort, maybe a bad TPS or MAP, maybe even a bad upstream O2 sensor.

I doubt it's vacuum related at all. Usually vacuum problems simply cause a high idle.

 

[JP98]

It shouldn't be the IAC, because it's fully bypassed by the butterfly valve when driving around. Although, cleaning it never hurts of course. But, it shouldn't ever make a difference when you're pressing the gas.

Have you checked for any codes? My guess would be a sensor issue of some sort, maybe a bad TPS or MAP, maybe even a bad upstream O2 sensor.

I doubt it's vacuum related at all. Usually vacuum problems simply cause a high idle.

I wouldn't have thought the IAC either until I cleaned mine last month and it cleared the issue up for me.

 

[Chris S]

It shouldn't be the IAC, because it's fully bypassed by the butterfly valve when driving around. Although, cleaning it never hurts of course. But, it shouldn't ever make a difference when you're pressing the gas.

Have you checked for any codes? My guess would be a sensor issue of some sort, maybe a bad TPS or MAP, maybe even a bad upstream O2 sensor.

I doubt it's vacuum related at all. Usually vacuum problems simply cause a high idle.

I have not checked for any codes. I will make sure to do so. Even though its probably not the issue, like you said it doesnt hurt, i will clean the IAC.

 

[Brantley]

I wouldn't have thought the IAC either until I cleaned mine last month and it cleared the issue up for me.

What was your issue? When you press the gas, the air completely skips the IAC so it shouldn't make any difference when driving. Sitting at stop lights and stuff like that (idling in general) is the only time the IAC should ever make a difference.

 

[JP98]

What was your issue? When you press the gas, the air completely skips the IAC so it shouldn't make any difference when driving. Sitting at stop lights and stuff like that (idling in general) is the only time the IAC should ever make a difference.

It was only at start up after setting for a long period. Just like the OP it would not want to accelerate and buck or as the OP said dogs. Once I went say 50 yards it would clear up and be fine. Cleaned IAC and it went away.

 

[Chris S]

It was only at start up after setting for a long period. Just like the OP it would not want to accelerate and buck or as the OP said dogs. Once I went say 50 yards it would clear up and be fine. Cleaned IAC and it went away.

Hopefully I will have the same luck with the IAC. I have searched several forums trying to find someone with a similar/the same issue. Glad there's a tj specific one like this!

 

[Jerry Bransford]

Does it have the problem right after you start it and the engine hasn't yet started to warm up? Is there any difference in the problem when the engine is stone-cold vs. once it has started to warm up? I wouldn't think it's the IAC either since the IAC only functions when you're not stepping on the gas pedal, when the engine is at at idle rpms.

 

[Chris S]

Does it have the problem right after you start it and the engine hasn't yet started to warm up? Is there any difference in the problem when the engine is stone-cold vs. once it has started to warm up? I wouldn't think it's the IAC either since the IAC only functions when you're not stepping on the gas pedal, when the engine is at at idle rpms.

Its honestly super odd, sometimes it will do it stone-cold sometimes it will do it after lets say i turn it off for an hour or less. Im going to clean the IAC anyway as it probably hasnt been cleaned in 20 years. Then i will go from there

 

[LegerV.]

Jeep for Reference: 97 4.0l 5speed manual

Recently (past few months) my jeep has been doing something quite odd... When starting up the jeep and driving it for the first couple minutes it completely dogs. It acts as if there is not enough fuel for it to get up to speed when in first, second, and third, until I have to basically gun it and almost hit rev limiter just to get up to the speed limit required on the roads nearby. The issue then vanishes after several minutes of driving and its completely fine no matter the gear.

We initially figured it must be the fuel pump or fuel filter. We replaced both and that didnt solve the issue. We took it into a shop and they replaced the distributor cap and did normal maintenance on it. Spark plugs are all good and still the issue. My only guess would be vacuum lines.

The vacuum lines are 20 years old so i would understand them being porous and/or having some sort of issue. Since it seems fuel related I have narrowed it down to the vacuum lines going into the charcoal canister. Starting/running the car with those vacuum lines on or off makes no difference.

Could it be that when I start it up there isnt enough vacuum and its not receiving enough fuel, and its not until the fuel lines are full of fuel that is able to hold substantial vacuum? Leading to me just having to replace the vacuum lines going from the fuel line/fuel tank into the charcoal canister?

Has anyone else had this issue or something like it?

Any ideas?

Hello man.

I have the same issue in my TJ 2006. I thought it was IAC or TPS, i replaced both but same issue again. I need to replace pump gas and filters, i will let you know if this stop the failure.

Same as you, when i start the engine and driving it, it loses power in 1st, 2nd and 3rd, this is like a "little gap" losing a bit power while shifting from 1st to 2nd, then 2nd to 3rd, right? The issue vanishes when the engine gain temperature but sometimes appears again, not often in my case.

The other issue i found is the idle. The RPM´s goes up and down when the car is stopped, this happens only when i push pedal to accelerate, goes down first, then goes up.

 

[JP98]

Mine was only when it had set for an hour or two or say overnight. It was like it wasn't transitioning from IAC as I stepped on peddle. It never happened once it was the slightest bit warm. If I would rev the engine before trying to take off it wouldn't happen but if I started up and tried immediately try to take off. The last time it did it my wife and I had went in to eat dinner at a restaurant, we came out and climbed in the Jeep, and there was another Jeep pulling up so I started up and tried to back out quickly to give him my prime spot up front. When I backed out it was okay because I was at idle but I stepped on the gas to move forward quickly and it bucked and dogged and I got up the next morning and cleaned the IAC. No problem since.

 

[Jerry Bransford]

Its honestly super odd, sometimes it will do it stone-cold sometimes it will do it after lets say i turn it off for an hour or less. Im going to clean the IAC anyway as it probably hasnt been cleaned in 20 years. Then i will go from there

It certainly can't hurt to clean it. No need to actually remove the IAC to clean it. After removing the air intake tube, start the engine and with it at idle rpms, spray a goodly amount of throttle body cleaner into the throttle body. The IAC will suck the cleaner in through itself and, likely, kill the engine due to it being flooded with cleaner. That's ok, just restart the engine and do it again. I like to get something like a third of the can through the IAC. This also gets the orifice in the throttle body clean that the IAC's regulating air plunger moves in & out of.

 

[Chris S]

Okay sweet thanks

 

[Chris S]

Update: I cleaned the IAC and the some of the throttle body. Seems to be happier now. The real test is later when the engine is cool.

 

[CMTJ]

The pic below shows the vacuum line I bumped off when I did my kids valve cover gasket. Jeep would barley run and spit. Check this line ma

ybe and look for wear spots or make sure its in all the way.

 

[Chris S]

mine appears to be intact, if i run into any more errors ill be sure to dive deeper into any possible vacuum lines issues. Im also going to check for any codes.

 

[Chris S]

Update: drove it and on cold start still did the weird lack of power and kind of pulse feel. Not sure how else to describe it. I cleaned the IAC with it still on, im going to try to do a more thorough job tomorrow just to make sure it isnt an issue. If my issue still persists I will check for a code and then narrow it down from there.

 

[Chris]

Thanks for the update, let us know what you find!