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

buildingdoc · Mar 7, 2013

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 · Mar 8, 2013

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 · Mar 8, 2013

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 · Mar 8, 2013

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 · Jun 13, 2016

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

steelnewfie · Jun 13, 2016

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

the bus · Sep 24, 2016

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

1955moose · Sep 24, 2016

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 · Sep 24, 2016

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 · Sep 24, 2016

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

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

buildingdoc

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