Hi,

Don't know exactly how to explain this proble, I will try my best:

Normally, when I start my truck, it idles nice and smooth (600 RPM I think)

Once in a while, after I start it, it's smooth for a few seconds, and then the idle starts to go up and down on its own, like its surging or something. As if it's going to stall, then it idles hi for a sec, then real low, and it keeps going up and down in 1 seconds intervals.

When this happens, I normally just step on the gas to get the revs up, then it goes back to a smooth idle.

I first noticed this when I had 600 Kilometres on the truck, but it was so subtle, I never payed much attention to it because it was very rare.

I have 18,000 Kilometres on it now, and it's doing it a tad bit more now.
It only does it at start up, while it's in PARK.

Also, When I first fire it up, if I turn my heater fan on full, this seems to make it surge again, until I step on the gas to get the revs up.

Not sure if I explained this well enough, but if anyone has any ideas, I would really appreciate it.

Thanks
Alex

 

I have not experienced this but I'll take a guess. The high/low searching sounds like your computer is trying to get the mixture settled out. An O2 sensor, air mass sensor, a temp switch, or bad connections in those wires can cause some of that. Starting your heater fan (20 amps?) at low idle could drop the battery voltage by almost a half volt. The O2 and air mass sensor readings can be affected by the lower voltage.

I would check and clean the battery cable connections (esp. the ground on the engine block), and the connection on the O2 sensor. Are your pulgs in good shape? Spark plugs are ALLWAYS a good thing to replace and can solve a lot of mixture problems. I would also check and clean the connections on the engine temp sensor, the air mass sensor and the engine crank sensors if you can at them. It may not fix the problem but it will eliminate a lot of possiblities. What year and what engine do you have?

 

Hi,

Thanks for your reply, I will try your suggestions and see if it helps.

My truck is an 2002 with the 5.4L 4X4 Supercab.
Diablo Chip
AF1
Gibson Supertruck

Approx 11,000 miles on odometer now.
Truck is always garaged, squeaky clean and wiped with a diaper.
Any other ideas are welcome.

Thanks
Alex

 

I have a 2001 Supercrew with the 5.4L. Now that I think about it, I did notice my idle searching around 4 or 5 times over a period of about a month. The action was fairly subtle and it stopped so I quit worrying about it. I think I will blame it on bad gas or damp weather conditions.

With so few miles on your truck, I will retract the statement about changing the plugs. That could wait unless the problem is severe.

 

Habibi: My truck does the same thing any ideas? Did you fix it?

Mike

 

Next time this happens, tap on the IAC with something like a screwdriver and see if the idle goes back to normal and even. If the engine idles fine after tapping the IAC, this indicates that the valve is sticking. Seems pretty common the the 5.4L.