Rough Idle

[mwake83]

I have a 88 Ranger with a 2.9l. It has developed a rough idle over time (the idle bounces constantly from 900rpm-400rpm). It will usually idle fine when it is first started but will start having this problem as soon as the engine is revved up. I have no codes. Parts I have replaced recently with no change include: plugs, wires, cap and rotor, map sensor, 02 sensor, coolant temp sensor, fuel pump and filter (I have 35psi fuel pressure) and I have checked for vacuum leaks.

I tried replacing the idle air control valve today and it makes the problem worse, i.e. the engine just dies when coming to an idle. But when I start it again it will idle perfectly until the engine is revved again. I have tried 2 different, new IAC's and both do the exact same thing. It seems to run best with the original IAC which has 235k miles on it. Any suggestions? Thanks in advance.

 

[mwake83]

Update: I tested for codes again and did have a KOER code 41 (system lean).

 

[mallards87]

I had this same issue with my 87. I ended up making a IAC block-off plate. Just drill a very small hole in the plate ( i started at 1/16 in.) where it covers the IAC inlet port on the intake. Gradually step up the size of the hole until you get the idle speed that you want. Remember that the slightest change in the size of the hole makes a big difference in idle speed. I believe I ended up with a 3/16 hole for mine to idle at 900 rpm. I haven't had any idle problems since.

 

[mwake83]

FYI in case anyone else has this problem in the future. I ended up finding a broken wire going to my o2 sensor which was leaning out the engine. The IAC was also plugged and not helping the situation. Fixed both and seems to be running great. Thanks.