At my wits end...96 Cobra idle and other issues...

LuckyDSquared

New Member
Dec 18, 2019
3
0
1
38
Maypearl, TX
Bought a project 96 Cobra a couple of months ago. Was running rich (codes) and seemed to have a pop or miss at idle and a slight miss while under slight acceleration. The car used to be supercharged (could tell from the Vortech coolant crossover).

I removed the SCT chip, swapped IMRC deletes to factory IMRCs (verified motor is working correctly), as well as general tune up (Motorcraft plugs, wires, and coil packs). Did a compression test and all seemed fine. I also swapped off-road h-pipe for catted x-pipe for inspection purposes.

The miss seemed to disappear other than a slight “pop” at idle. But after a good 30 min or so drive I got on it at it studdered through 1st and had a slight hiccup under normal acceleration. But it was intermittent. Haven’t taken on a drive since then.

The car still idles High. 1800 and maybe 2000. Will not drop down. I’ve verified the TPS voltage and MAF voltage. I replaced the IAC valves. Both the factory valve and the replace perform similarly (Idle is solid 1800 and will not come down even if IACis unplugged). However, one IAC the parts guy told me fits a Mercury. That IAC brings the idle down to 1200 for a couple of seconds and then to 750 or so. Sometimes will idle irrationally 750-900 bouncing around. But at least it doesn’t idle at 1800-2000.

I just replaced engine grounds and battery leads. Was messing with it today and now it has a rotten egg smell in exhaust. Fuel pressure showed 30 w/vacuum and 40 w/vacuum but it took a minute to build up. It also wouldn’t hold more than 10 psi with the key on but not running.

Have done a homemade smoke test and don’t show and vacuum leaks. Anyone have any better ideas before I light a match?
 
  • Sponsors(?)


LuckyDSquared

New Member
Dec 18, 2019
3
0
1
38
Maypearl, TX
So today I backed off my throttle body set screw and the car still idled at about where is should 750Ish. that’s with the IAC unplugged. I pressed on the throttle linkage to force the throttle blades closed a bit more and the car died. I guess the throttle body needs a cleaning?

also, I drove the Car for a good bit and the car didn’t miss much. But, it would have a slight, but noticeable, hiccup on normal throttle pads or slight acceleration. No CELs, new plugs, wires, and coil packs.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,849
505
204
Houston Texas
750 RPM's with the IAC unplugged is still too high.

Troubleshoot IAC idle problems 1996-2004

The fact that butterfly doesn't fully seal is also an issue. If the throttle body is dirty a light cleaning is advised.

Do you have an ODB2 scanner? If you did it might provide some very helpful information. Knowing what the TP-MODE PID is will confirm if the TPS is working as it should.

ForScan ODB2 scanner w ELM327 USB