Weird idle issue

91 240 auto. I?m having a strange issue with my idle that I can?t figure out. At cold start up the RPM shoot up and then quickly fall down to where the engine almost stalls then idols normally. Once it warms up and goes into close loop the idle is quite high I don?t have a tac but I?m gauging it around 1500 RPM. The strange part is when I check for vacuum leaks I can remove vacuum lines from the manifold and the idle does not change. I?ve done this with the fuel pressure regulator line and the HVAC line. I can feel the suction so I know they?re not clogged. I recently replaced my failing 556 computer with a 946 and I am using it in conjunction with the 855 EZK. I?ve been reading that you shouldn?t really mix and match computers and EZK?s? Some conflicting info there.

i had/have? a similar issue with my 92 240. it would only happen on a warm engine. my fuel pressure regulator seems to be functional as well. this past weekend i replaced main fuel pump and filter, the PCV system, and cleaned the MAF meter and the IAC. it now no longer does the dip after surging upon start up but stays high at 1000rpm, so my next test is of the Idle Air Control Valve to see if it can settle to the appropriate 750rpm range. i tried to do the diagnostic of clamping off one of the IAC hoses but couldn't get a good angle, but the half squeeze i could do didn't change the idle at all, so i'm thinking it's not functioning. my suggestion, check your fuel pumps and the IAC. also take note if this is happening when your tank is more or less than 75% full, that may indicate it's the in-tank pump.

disclaimer: i'm reasonably new to this, but i've been reading <i>a lot</i>

seanile said:

...stays high at 1000rpm...

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If no vacuum leaks, adjust throttle plate to OEM spec...always start with throttle plate adjustment.

So that's where i kinda arrived at this eve. When i had the MAF off last weekend the throttle body was definitely fouled by oil from my ****ed up PVC, but i wasnt able to take it off to clean (no gasket on hand, i get one delivered on Thursday). What i did today was clamped a hose on the IAC and had no change in idle. Then i ran the diagnostic procedure here (http://www.swedishbricks.net/700900FAQ/Engine-Codes1.html) and it was able to actuate the IAC, and showed no codes. I then manipulated the throttle body manually while the car was off and heard no soft click.

So my presumption (and ultimate question) is if the throttle body doesnt close due to gunk, does the sensor receive that as the throttle body being engaged, and therefore the IAC can't actuate? I was assuming the throttle body needed some other input in order to disengage the IAC.

Additional question, do all throttle bodies have an idle adjustment screw on their underside? Because mine may have fallen off...

Sorry for the hijack

edit: more reading (got me here) in which it says the 2.4 and 3.1 systems for 1992 set the idle via the ecu and a small set screw, rather than the 2.0 and 2.2's butterfly adjustment screw. so cleaning the throttle up nice and making sure it's set-screw allows it to close flush ought to solve it. i'll edit this post again for posterity if it does not succeed.

seanile said:

(no gasket on hand...)

Click to expand...

Silicone grease (dielectric) works just fine via greasing old gasket.

>PVC

Check vacuum line going to flame-trap. Mine was plugged at intake's port fitting

>throttle body...heard no soft click....dirty/etc

See LH manual...Google search

< idle adjustment screw>

Early LHs did, but I don't think this is found on LH 2.4

thanks! I replaced the whole pcv system so we're good there. my money is on the throttle body at this point because i know it's covered in oil from the previous pcv system. New gasket come later this week with new IAC hoses for good measure.

seanile said:

throttle body... 1000rpm...750rpm...

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When an upward change in idle RPM happens, extra air entering intake is the culprit.

If throttle plate is returning to its set position, then fat chance it is the issue.

As throttle body wears, over a long period of time/usage, then its shaft at body will allow small amounts of air to enter. But, idle RPM is not going to jump from 750 to 1,000 RPM the next day. I suspect a carburetor shop, very few these days, could fix this issue, but I'd find an used throttle body.

Sidebar - On LH 2.2 and before, distributor set incorrectly (timing advance issue) can cause higher idle RPM...on LH 2.4, extra air. With a Crank Position Sensor (LH 2.4), timing is controlled via ECM, but on LHs before, initial timing setting is controlled by whoever sets it.

Air leaks are a common issue. Spray around with brake cleaner and see if the idle changes. Another one is misadjusted TPS. Another one is speedometer inop. Idle issues can happen if your speedo is inop.