alright i let my truck sit for about a month or so while i waited for parts for it. i've been driving it for a couple days now and today i noticed that when it sits at idle it will rev itself up to around 1700rpm's and go back down to low enough rpm that the motor starts to shake. i also noticed that when it was happening my oil pressure dropped quite a bit. i turned the truck off and turned it back on and my oil pressure went back up but after sitting for about a minute the motor started to rev itself up again. is this a TPS sensor starting to go out? or IAC? i was even thinking it could be a clogged cat or something. tell me what you guys think.

 

model? year? engine?

 

1990 f150 with 300 inline 6.

 

welll.......im not certain but you might wanna check the idle control motor..my ranger did that just before that motor went out...clogged cat..dont think so...mass air flow sensor maybe..not to sure about this one man

 

hmm no experiance with those myself...someone will chime in and help you out im sure

 

I had a similar problem but not quite that bad.. just a lot of changes in the RPM while im just sitting there... My throttle body was caked in black coal looking stuff... I gave that a good cleaning, completely replaced the IAC valve and started it up and it was perfectly fine. Check those, just make sure if you take them off you buy new gaskets for them at the ford stealership.

 

I didn't think the '90 would have a MAF? (That's the sensor that sits in the air intake, right?)

I would recommend reading what, if any, codes are showing on the computer. I have a '92 and I'm pretty sure it doesn't have a MAF (unless I'm thinking of something else that didn't appear til later models)

Check for broken or disconnected vacuum hoses, or whether the a/c is turned on when it does that, my truck idles in a similar way when the a/c is on (although I am having lean issues on combustion. probably separate issues)

 

ohh wow i must have looked over the 1990 year model thing lol..no a '90 wont have a MAF