Random idle... goes from 600-800 every second

[buildingdoc]

Okay.. so here are the conditions.

2004 Expy 4.6. Engine warm either P or any gear. engine surges to 800... sometimes more, then "relaxes" back to about 600 rpm. when it does maintain a smooth idle about 40% of the time I get a humming/fog horn noise.

what I have done. Checked and replaced necessary vac lines. All spark plugs good and seated. fuel filter has about 2k on it. K and N filter cleaned and recharged. IAC changed. MAF clear. no Codes... changed only my bank 2 Catalytic converters.

I am thinking the random idle is due to only one side cats being changed is having the O2 sensors giving different readings to the comp to cause the constant rpm fluctuation.

Any thoughts before I get 4 new O2 sensors and do my Bank 1 cats?

TIA

Matt

 

[ELVATO]

Which vac lines did you replace? You checked the elbow behind throttle body, yes? It kind of sounds like there might be a small leak and your IAC is constantly trying to compensate.

 

[1997SCEBFEX]

Sir,

sure your troubleshooting led you to this, however the pcv valve usually 'whistles' if loose/not properly seated etc.. agree w/ above post in that you're getting unmetered air somewhere in the flow and the sensor(s) are trying to compensate.

PanHandler.

 

[buildingdoc]

Both PCV and the line to the rear of the throttle body were replaced as well as another vacuum line on the right side near the brake booster. After those were replaced my codes went away and everything was fine until I had a Cat start rattling on me about 2 months later. Everyday I leave for work 20 minutes early so that I can pop the hood and do a once over on the engine. Maybe take it to a shop to get a vacuum test done on it. whatever it is it definitely gets larger as the engine warms up.

 

[0xowner]

Did you ever find this out? I have the same thing occurring

 

[steelnewfie]

Run a can of seafoam or fuel system treatment. Helped out my intermittent idle and misfire problems.

 

[the bus]

My guess is a leak at the intake. Spray some carb cleaner around it to find the leak.

 

[1955moose]

Vacuum leaks will always get worse as the vehicle warms up, and fuel trims lean out. When cold the computer and sensors give your engine a richer mixture. As stated above you'll have to check for intake leaks, around gaskets, hoses, anything involving the fuel/ intake system. You might want to run a scanner on it to check dtc codes. Keep us posted what you or your mechanic find.


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[Bedrck47]

Again another thread in which the OP hasn't been back to his problem in over 6 months.

OPPS 3 years not 6 months

 

[1955moose]

6 months my eye, didn't notice it's been 3 years. Thought I was slow with things, jeez.


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[Habbibie]

Well since this thread has been bumped I had a similar problem and when I replaced my VCT's it fixed it.

It wasn't the cheepest fix nor was I actually intending on fixing the idle jumping by replacing the VCT's but for some reason that got fixed alone with what I was trying to originally fix

 

[Transporter]

Now now guys, 0xowner is doing things the correct way. He searched the Forum first to see if somebody was having the same issue he was having and what solution they found BEFORE starting a new Thread. He revived an old thread to see if the OP had finally found a solution.

0xowner: most of these issues are from unaccounted for intake air or incorrect vacuum IE a leak that the computer keeps trying to compensate for because the computer has to base its monitoring and corrections on the assumption that the system is properly closed and functioning.

 

[1955moose]

Also an item that causes surging is a bad throttle position sensor. This usually will trip a check engine light, but not always. As stated above spray or use propane and check all intake gaskets and hoses. Also run a dtc test through your data port for any codes. One failing or failed sensor can cause havoc.


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[Bedrck47]

Now now guys, 0xowner is doing things the correct way. He searched the Forum first to see if somebody was having the same issue he was having and what solution they found BEFORE starting a new Thread. He revived an old thread to see if the OP had finally found a solution.

0xowner: most of these issues are from unaccounted for intake air or incorrect vacuum IE a leak that the computer keeps trying to compensate for because the computer has to base its monitoring and corrections on the assumption that the system is properly closed and functioning.

Your reply to oxowner may not do any good He hasn't been on the forum since Aug 8, 2016

 

[1955moose]

He might not be but the info might help another member with similar problems.


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