Need more help and advice guys on my cursed 90 Mighty Max with a 2.4L.

As most of you read in other threads throughout the past year, I got this thing cheap, solid - but ran like crap. Had evidence of a coolant leak on #4, so I pulled the head to replace the head gasket.

Had the had cleaned and they found a crack between #3 and 4...so new Clearwater Head. Has the injectors ultrasonically cleaned. Cleaned out the entire intake of every bit of carbon, including all the corners to (hopefully prevent future buld-up. Cleaned out the EGR and bench tested it OK. New cooltant sensors, new O2 sensor, new throttle body (mechanic tested original one had leaks) new exhaust incl. new CAT and new exh manifold.... yet it STILL Ran rough - almost misfiring.... ran it like this for 10min and exhaust began to superheat. So bad in fact the space between cab and bed was too hot to touch and new muffler discolored slightly. Getting black smoke and black soot out tailpipe and was still getting 6mpg avg when I did drive it short distances. Basically running rich.... super rich. But it ran with a good idle rate.


Opended up ECU and had the blown capcitors replaced by AvPro, juat got it back today and installed it.

Now guess what... It actually runs good, no more steady black smoke and "misfire", old chunks of soot being blown out clean....but the thing is running at super high rpm cold.

No tach, but I'd guess it's at least 4 to 4500rpm. Had helper verify on startup the throttle cable is not moving at all and has no tension (slight slack). Slight tap of gas pedal adds another 500rpm or so. Dangerously high for a cold startup I think.

So...do I start looking at and idle speed motor, or idle speed postion sensor, throttle position sensor, etc even though the thing ran normal rpm before the ECU repair?

Or does anyone think AvPro screwed up my ECU?



Jim

idle speed motor,

Swapped out the idle speed motor... no change. Tried the idle speed motor sensor... no change. TPS... no change.

I'm really suspecting at this point the ECU repair might have been botched by AvPro. Especially since I did not have high reving rpm's on startup until after the repair. Need to locate another known good ECU to verify though.

FSM is a little confusing. Refers to both idle speed motor and motor position sensor, but diagrams of each show exact same thing. I think the editor fell asleep. Isnt there a position sensor of sorts inside the distributor as well?

Any other ideas from the group?

Truck still races up to 4000-4500 rpm immediately at startup.

Coolant sensor is brand new. as is most of the upper half of the engine.

Jim

Does your TB have a wax pellet style FIDLE valve like the 3.0 in addition to the isc?

Has the idle bypass screw been fooled with?

Has the mechanical idle stop screw been molested? What is the idle stop opening (measure with drill bits, gap from buttfly to bore wall)

dont believe it does. For a TB, it's very very small and very very basic. not much to the damn thing once you remove the idle motor assembly. that's the other thing ... with either set of idle speed motors and position sensors, neither motor is engaging the butterfly. something else is revving the thing up.



It's entirely possible - although I havent. At one point my mechanic had the thing before I did the exhaust (complete incl. manifold to tailpipe) to recharge the AC, asked him to diagnose rough idle.... he said original throttle body had a bad vacuum leak. Got another one from a maxd50scene forum guy, he installed it for me (travelling on biz at the time) said that one was good. he may have adjusted the idle screw to the extreme, which at the time the truck still had a bad ecu... so we were fighting a bad ecu at the time. Ignoring what the wonky FSM might say - where should I start on the screw? about 50% point of range? back it all the way down and maybe start low at 20 or 30% of max? never really had to mess with those screws so I'm a bit green in that area... where it should be, how much it alone can impact the rpm's (all the way to 4500?)



She's cracked open a bit. When i last looked at it Tue night, I'd say maybe it was cracked open about 1/8" or so. About the same as original throttle body on my shelf actually. Will recheck this and maybe also play around with the idle screw..

jim

I'd take a 2 step try to set that stuff right.

Crank it, and let it idle. See if the idle drops as it warms up. High speed idle won't hurt it if no load.

If it doesn't drop, back off on the idle stop screw on the butterfly and see if the idle drops. Back it all the way until it no longer contacts the idle butterfly arm unless the engine stalls out. If it still idles too high, then start closing the idle bypass screw until the idle drops into the desired range.

I think the idle butterfly opening of 1/8" is too wide, and may be enough for the idle speed noted.

After you do the above, you should reset the idle stop screw to the proper butterfly opening (I'd consult the fsm) and then adjust the base idle speed with the bypass screw (probably between 600 and 750rpms) with the isc disabled, then readjust the tps sensor.