dbull

I'm trying to eliminate a low cruise miss(1800-2200rpm) on my 91/5spd f150.New plugs,wires,fuel filter all helped but I still have a miss/buck while cruising at 40-50mph, while acelleration is excellent.When I pulled the connecter off of the iac valve, idle speed stayed at 1100rpm and with it blocked off it idles smoothly at 900rpm which indicates a vacuum leak, but where?I have sprayed all hoses and connections w/carb cleaner with no change in idle speed and I'm now thinking maybe a valve in the emissions system is stuck. Any input would be greatly appreciated!Thanks dbull.

ymeski56

There are three motor conditions that come to mind that seem to be the last to be considered &/or addressed that are rather common & are probably responsible for more replacement parts being thrown at an engine in the attempt to rectify some abnormal behavior to no avail. 1. Low &/or unbalanced cylinder compression. Where someone is trying to beat a dead or lame horse into obedience. Nobody likes to do dry/wet compression checks because if you've ever done one, you know there a pain in the ***. But a necessary one. It's the only way to get an idea of the general health of the motor & weather you might be throwing good money after bad and end up having little to show for it but a drawer full of receipts. You can increase your MPG & performance simultaneously by bumping your timing. Doesn't cost a dime. But if your cylinder compression is low or isn't balanced within acceptable range of variance, you better have Triple A on speed dial. 2. Hanging &/or clogged injectors. Well known to screw up an otherwise perfectly good Idle. This and 3. Combustion chamber deposits which wick fuel resulting in abnormal detonation behavior and an artificially increased rate of compression due to reduction in cylinder c.i.'s, displaced by deposit build up. There is no reason to believe that this deposit build up occurs in all cylinders at the same rate. There are no sensors, valves, solenoids or relays that will rectify any of these preexisting conditions. My compression is +- 2 psi between all cylinders. & I'm running 16BTDC, 82k on a 302 ci motor. 17/23 mpg. & great performance. I toss in a bottle of "Lucas Injection Cleaner" every 5-6 tanks & a can of "Seafoam" poured into the air intake via the brake booster vac line w/ motor at idle, just before each oil change. These are the three things I addressed in order to cure the searching Idle my truck had when I got it w/68k miles at the time. If your not showing error codes on a KOEO or KOER scan, 1, 2, 3 or some combination are more than likely responsible. My suggestion is make sure your starting w/ a healthy engine to begin with, then decontaminate it, then see what you got! This is my personal opinion (& you know what they say about those) Comments are welcome. There are other considerations of course, but these most closely relate to Idle misconduct. For finding Vac leak, I prefer to poke around w/ an unlit propane torch set at a very low flow rate. Also check to see if your EGR diaphram is leaking vaccum through. As for the IAC, See this link:http://fordfuelinjection.com/?p=39

dbull

I ran an electronic leak detector around the intake and got a strong reading around the throttle body.Pulled it off and sure enough the gasket was sucked in almost completely.Cleaned and replaced and cured all my driveability problems!dbull