rpms stumble at idle

[Jibit]

I've had this problem for a while, not throwing any codes. Its worse when the car is in gear. It doesn't bounce as much in neutral or park. I found a vacuum leak a while back on the upper intake manifold but that didn't correct the problem. I had the hood open yesterday and could hear a loud hissing sound on the passenger side by the fuel rail but couldn't find anything. Is there a vacuum line that is routed under the upper intake manifold? Any suggestion on where to start?

 

[JoshMcMadMac]

Have you cleaned the EGR and IAT? They tend to throw codes, but are also typical culprits of surging idle. Is there a difference when you engage the AC? Have you set the idle or checked the timing recently? It could be any number of things right now, but hopefully we can narrow it down quickly.

 

[Jibit]

Have you cleaned the EGR and IAT? They tend to throw codes, but are also typical culprits of surging idle. Is there a difference when you engage the AC? Have you set the idle or checked the timing recently? It could be any number of things right now, but hopefully we can narrow it down quickly.

All I've cleaned is the throttle body and MAF sensor. I don't use the AC. How do you set the idle or timing with these cars, its controlled by the computer?

 

[Sergmark]

There is a rubber/metal line on the passenger side between the engine and firwall that rusted out on my 96. I had a rough idle from what I remember. :invasion:

 

[Jibit]

There is a rubber/metal line on the passenger side between the engine and firwall that rusted out on my 96. I had a rough idle from what I remember. :invasion:

Do you know what they connect to? I remember seeing two lines on the back of the engine that were rubber hoses out of the upper intake and then into metal lines. Don't know where the metal lines go though? Regardless, I don't think that was the problem but I can check again. Do you know if any lines run under/through the upper intake manifold. It has plenty of spaces by the fuel rails that they can be routed.

 

[UNVMYCTS]

Trying tightening the intake, that hissing may be air seeping through a loose connection. That would also cause a rattling at idle. It worked or me. Unfortunatelt it needs to be tightened every few months. Downfall to powerful engines.

 

[Topcat]

Dave
If you Hear a loud hissing noise and the idle is erratic , you probably have a vacuum leak .... Try this go buy a can of carburetor cleaner with the little red plastic tube ...With the car idling spray through the tube where you hear the noise ... when you find the leak the idle will change , probably increase ...keep moving around until you find the exact place of the leak ... It is possible to find even very small leak in this way , yours sounds like its pretty good sized.. also check where the air tube meets the throttle body , maybe loose or not seated properly also check that the gasket is present on the throttle body ...Keep us informed..
Note be careful not to spray the carburetor cleaner on the exhaust manifold as it is very flamible...

 

[Sergmark]

Dave sorry I don't know where the lines go.When I bought the car they took it to the dealer and spent 770.00 on miscellaneous parts so I could drive it home.
This is the part # of what they replaced f7lz-68899-aa Description saysTEE-crck vent. :Bang
I hope that helps.

 

[Jibit]

Dave
If you Hear a loud hissing noise and the idle is erratic , you probably have a vacuum leak .... Try this go buy a can of carburetor cleaner with the little red plastic tube ...With the car idling spray through the tube where you hear the noise ... when you find the leak the idle will change , probably increase ...keep moving around until you find the exact place of the leaK

What do you mean by spray through the tube where I here the leak? If I knew what tube had a leak I wouldn't be looking for it? Not trying to sound sarcastic just don't understand what you mean. Unless you're saying to try all the vacuum lines and when the idle changes that's when I know that's the line with the leak.

BTW, my brother-in-law has the feeling its the EGR. He thinks that the metal tube coming off of it may have rusted through. Anyone else ever have a similar problem?

 

[MarkedMan]

What do you mean by spray through the tube where I here the leak? If I knew what tube had a leak I wouldn't be looking for it? Not trying to sound sarcastic just don't understand what you mean. Unless you're saying to try all the vacuum lines and when the idle changes that's when I know that's the line with the leak.

BTW, my brother-in-law has the feeling its the EGR. He thinks that the metal tube coming off of it may have rusted through. Anyone else ever have a similar problem?

Hey Dave


When in doubt BURN IT

 

[driller]

I suggest starting by familarizing yourself with the vacumn lines...

The plastic fittings are the predominant suspects.

 

[Jibit]

Do you have any clue on what the lines going to the gas tank are?

I remember something snapping off the gas tank when I went to install the fuel pump but didn't know what it was so I never replaced it. I think it plugged directly into the fuel tank. This wouldn't be the cause of the vacuum sound on the top of the engine though.

Is there anything I can spray around the engine to find a leak, sort of like the dye they use for AC leaks or soapy water for the air bag leaks?

 

[driller]

You should have Canister Purge lines going back to the fuel tank. They are part of the EVAP emission control for fuel vapor control.

You are evidently looking for a vacuum leak. Us 'old timers' used very dangerous methods for finding vacuum leaks, such as carb cleaner, starting fluid, propane... etc. Propane was always my favorite. The idea behind all these was to use it as a 'scanner' and when the engine vacuum picked up the extra 'fuel', you would hear and see the idle change. This worked very well for intake manifold leaks, carb gasket leaks and the like. I liked using the portable propane torch with a length of hose over the end and use it to 'sniff' around the engine. When you got close you would see the rpms pick up.

Of course I would never recommend that someone use propane around the engine compartment of their Mark VIII. Far too dangerous and I'm just lucky to be alive to tell the tales.

Nowadays, the kids they call mechanics have some kind of 'smoke machine' to help locate such leaks. Wusses. :tongue:

 

[Jibit]

Well most of you don't know me enough to know that I'm a pyro! I had my first problem at about 10 when I let a couch on fire, in my house. Oddly enough I work for a company dealing with fire alarm control panels. However I am still a pyro and might just have to try the propane, considering its getting close to the 4th and all!!!:gr_devil: :zgreenbou

 

[MarkOfDeath]

Try replacing the IAC ( idle air control )

 

[Topcat]

Hi Dave
The little plastic tube is on the spray can , and directs the flow of carbcleaner in a narrow stream at all the vacuum hoses till you find the change in idle.. you can spray around your intake also the little plastic trees with multiple hoses going to ...I am sorry I didn't explain myself better , I have never set anything on fire doing this procedure but I feel safety should come first ...You will do no damage to anything as long as you use a little common sense..As John Driller has said there are other methods to achieve the same results I just prefer this one...ther are a lot of little plastic tubes in the the vacuum system that get brittle with age and break very easy ... They can be repaired by putting the two pieces back together with very small vacuum line or Windshield washer hose...

Dave if you are not comfortable with procedure ,don't take a chance ,take it to a mechanic for the repair... Please it is very costly to just replace parts without knowing ...

 

[Jibit]

Dave if you are not comfortable with procedure ,don't take a chance ,take it to a mechanic for the repair... Please it is very costly to just replace parts without knowing ...

I'm not afraid to try anything once! I won't go the route of replacing components, its not worth it. Besides won't most of them throw codes? I'll try to work on the car this weekend and let you guys know what I find.

 

[Black87LSC]

Try replacing the IAC ( idle air control )

Jibit, I hear ya... I got the same problem. I've been checking all the grounds in the car lately. It's really annoying!! :soapbox: :Bang It stalls in traffic sometimes, and other times it will run perfect at 750-800 RPMS. If I run the A/C it will idle at about 600 RPMS but will sometimes stall without any warning/surging! I bypassed the IAC just to test it and that didn't make it much better.

 

[MarkVIIIDylan]

ok i heard that same hissing noise and thought it was a line leak somewhere. What i found and as was posted only a few times at Markviii.org was that there is a hole on the underside of the throttle body. if you take it off and look you can see the hole is threaded. nobody seems to know what the heck was in there but people who have experienced the same problem have just jammed things in there to plug it up and stop the noise i just took my TB to a local plumbing supply and had them find a stubby bolt that matches the threads. i used some silicone to create a sure seal and to help keep it in. i didn't torque it down incredibly tight just silicone and a little more than hand tight. also make sure the bolt is not too long since the idle air passes directly above this opening. If anybody knows what this hole was used for please enlighten me because even the ford manual says nothing of it.

 

[Jibit]

What i found and as was posted only a few times at Markviii.org was that there is a hole on the underside of the throttle body.

That would make sense, considering the sound is coming from the top of the engine. This weekend I tried using carb cleaner to find the leak, nothing. I removed the upper intake manifold to see if I could find the leak nothing. I tightened the bolts to the lower because they were a little loose (and snapped the head off a bolt because I'm stupid and don't know my own strength).

The next day my CE light comes on and a code spits out P0174 (bank two lean). I'm assuming this is a O2 sensor. Also, I'm starting to wonder if problems are being caused because when I replaced the fuel pump something broke off the fuel tank (I'm assuming its the vacuum line everyone has been talking about) but no codes were ever given from it.

 

[Topcat]

Dave
bank 2 is lean ,could still be a vacuum leak ...Air drawn in from a broken hose or gasket , causes the hissing noise you are talking about and the extra unmeasured , uncontrolled air in the intake causes a lean mixture..I am not sure which side is bank 2 ,I think the passenger side but I am not sure, Ill still bet there is a vacuum leak there..I cant believe you can hear it , and carb cleaner didn't work..Did you spray around where the inlet tube meets the throttle body , Anyplace in the air tube behind the MAF has vacuum in it , but that should affect both sides not just bank 2...

 

[Jibit]

Did you spray around where the inlet tube meets the throttle body

I never thought of spraying near the throttle body. I only sprayed around the hose that connects to the top of the intake tube just before the TB, I think it runs back to the valve cover. I was mainly concentrating on the intake, hoses connecting to the intake, and anyhing with plastic fittings behind the intake. I'll go home and try the TB and see what happens.

 

[Jibit]

Well, plugged the hole on th underside of the TB. The car smoked a little when I first started it but that may have been from spraying it down with carb cleaner. Drove it to work today and the rpm's are stable again. I can still hear the hiss, probably because the bolt I put in didn't fit properly. We'll see after I drive it home if it changes. I worried about the hole. What goes there and did it fall out or get sucked up into the intake?

 

[Topcat]

Dave...
Koodos to MarkVIIIDylan for an excelent piece of information , but if you still hear hissing you are not out of the water yet ... Sounds like you might have found your major problem but there should be no hissing on top of the motor..

 

[Jibit]

Dave...
Koodos to MarkVIIIDylan for an excelent piece of information , but if you still hear hissing you are not out of the water yet ... Sounds like you might have found your major problem but there should be no hissing on top of the motor..

John,

The hissing there but not as loud. Like I said, I think its because the bolt didn't fit properly, I'll have to size a bolt and try it. I'm just curious if the CE light will turn off now.