Idle speed oscillates - why?

Messages
682
Location
ontario canada
I have the Mitsubishi Mirage Colt 1995 with the 1.5l 12v sequential fuel injected. When it's cold the idle speed goes UP and Down, and UP and Down, etc, etc until it gets fully warmed up. When warmed up, it still seems to idle too fast, because I don't get the amount of engine braking with my foot off the throttle, and when I put it into neutral at a stop, after about three or four seconds the engine slows down to a reasonably slow idle. I'm taking it in tomorrow, but first I'm hoping for an opinion about the problem. I have 3 Colts, and two of them suffer from this problem. I've tried lots of fuel injector cleaner, Berry Chemtool, acetone, with no help. Do I just need to order some rebuilt fuel injectors? or a fuel rail?
 
Messages
35,993
Location
ME
The idle speed controller is the first suspect, it's an air bypass around the throttle plate. Depending on the car design you can run with some intake plumbing off and cover the idle speed control hole with your finger. Should slow the idle down or stall the car. If you're quick you can pull your thumb off to see if it overcompensates... then finds the right idle. These passages and motors can generally be cleaned with carb/tb cleaner. Other things it could be... could be going in and out of closed loop where open loop idle is faster which warms the o2 sensor, which brings the idle down... could try momentarily unplugging that. Could be worth watching the data stream if you can get at it. Does the car stutter or run poorly just off idle? If so your tune up hypothesis holds water.
 
Messages
25,045
Location
ON, Canada eh?
Mas airflow sensor... Seen this problem with a '89 2000 GTX that had a similar setup. Try disconnecting it and running the car and see if the problem gets better. The car will run but a tad rougher. It will be enough for you to see if this is your problem.
 

scoobie

Thread starter
Messages
682
Location
ontario canada
Yes, my mechanic was saying something today about it being something to do with airflow (I didn't quite understand it) - He said he was going to try to clean something (probably what eljefino was saying - clean the air bypass around the throttle plate) and i'll suggest he check the mass air flow sensor as well. Thanks very much guys!!! I really appreciate it!!! :)
 
Messages
25,045
Location
ON, Canada eh?
Good luck bud, PM me if you need more help, my dads a bit of an expert on these cars because we have had a few over the years. Colts, 2000 GTX's etc...
 
Messages
502
Location
Canada
I think the mass airflow is incorporated into the air cleaner. I seem to remember these having an adjustable idle bypass on the top of the intake. It required some tweaking with a Phillips screw driver. IIRC if the bypass was screwed too far out the idle speed motor would hunt because it couldn't bring the idle down enough. Often guys would fool around with the bypass screw to up the idle speed to mask a rough idle. The valves on these get tight and I fixed many (rough idle) back in the day with a simple valve adjustment. (very easy to do on these. re-torque the head bolts while in there too)
 
Messages
1,714
Location
CA.
Not to hi-jack a thread. I have a buddy with an old F-150, 1988 4.9L that does something similar. It surges when he first turns on the A/C, and hunts for an idle speed, then calms down if it steps on the gas slightly. It's annoying because its a stick shift. EGR and most of the senors are new including the throttle bi-pass valve. Any ideas? AD
 

scoobie

Thread starter
Messages
682
Location
ontario canada
I have located a cleaned and reconditioned and guaranteed idle control valve for $50 on ebay - I'll go for it, and take apart the old one and give it a good cleaning and keep that one on the shelf.
 
Messages
43,749
Location
Ontario, Canada
 Originally Posted By: ADFD1
Not to hi-jack a thread. I have a buddy with an old F-150, 1988 4.9L that does something similar. It surges when he first turns on the A/C, and hunts for an idle speed, then calms down if it steps on the gas slightly. It's annoying because its a stick shift. EGR and most of the senors are new including the throttle bi-pass valve. Any ideas? AD
O2 sensor(s). Not much to the EEC-IV systems, and only a couple things can cause surging.
 
Messages
1,714
Location
CA.
 Originally Posted By: OVERK1LL
 Originally Posted By: ADFD1
Not to hi-jack a thread. I have a buddy with an old F-150, 1988 4.9L that does something similar. It surges when he first turns on the A/C, and hunts for an idle speed, then calms down if it steps on the gas slightly. It's annoying because its a stick shift. EGR and most of the senors are new including the throttle bi-pass valve. Any ideas? AD
I passed the info to my buddy he said he replaced the 02 sensor along with the EGR valve,TPS, and IAC. The fuel injectors were cleaned through the Schrader valve with the 3M FI cleaner too. No codes, A/C season is coming and it is a PIA especially with the 5 speed tranny. Not sure if he used a genuine Ford MC 02 sensor though. O2 sensor(s). Not much to the EEC-IV systems, and only a couple things can cause surging.
 
Messages
19,479
Location
Chicago Area
Clean your throttle body bore and butterfly plate really clean. Then the idle air control valve. Be careful with strong solvents because it may erode/eat plastic parts. All vacuum lines[including the PCV system] and intake connections must be leak free.
 
Messages
10,597
Location
Nokesville, VA
On the Ford, there is a wire that signals the PCM that the compressor has been turned on. Maybe this isn't working. Run a Key On Engine Running test with the AC on and it should give a code saying that the AC was on during the test. If it doesn't..the PCM isn't being signaled that the compressor is running and therefore isn't compensating the idle for that.
 
Messages
43,749
Location
Ontario, Canada
Just an FYI: Ford EEC-IV has an issue with rich conditions. Not that it causes them, but rather, it does not deal with them well. Now, you can have a REAL rich condition (MAP sensor not seeing proper vacuum due to vacuum leak) or you can have a pseudo rich condition where a bad O2 sensor makes the ECM think the engine is rich and then the engine is on a never-ending quest for proper feedback from the sensor which causes it to dump and pull fuel, causing the idle to surge up and down and can even lead to stalling. While most of the time it is IAC or something stupid, you've addressed the most common causes which is why I brought up the vacuum leak possibility or the MAP sensor, which, if not functioning correctly, can trigger too much fuel and cause a similar scenario as to what I depicted above. Does it do it when cold or only when warmed up? I ask that because I have always been able to eliminate the surge on cars with baffed O2's by forcing OL by unplugging the O2 harness. Quick and easy diagnostic technique. Typically, the O2 issues only rear their head when the ECM tries to go into feedback mode, which never occurs with the harness unplugged. EEC-IV is not that complicated, but there are some problems that can drive you NUTS!
 
Messages
1,714
Location
CA.
 Originally Posted By: OVERK1LL
Check for a vacuum leak then. And maybe another MAP sensor.
I had helped my buddy check for a vacuum leak and found the EGR valve was leaking, fixed that for him, no more vacuum leaks. I'll mention the MAP sensor to him. I borrowed the truck, and it can drive you crazy. Here is what happens. Turn on A/C, engine just started, rpms go up and down, near stall, to about 1200 rpms, this repeats itself several times, then sometimes it doesn't do it. If its surging, give it a little gas its fine. Now go to take off, let up the clutch, rpms dip slightly computer brings up rpms. Then the driver brings up rpms, you take off with too much gas because the computer senses rpm drop and speeds up the engine. This happens while driver is giving it a little gas as well. This happens very very fast, and happens more so with the A/C on. But it can happen with it off as well. Sometimes the take offs are perfect, other times the engine takes off with too many rpms. Very annoying, engine can be hot or cold it doesn't matter. Its almost as if the driver doesn't know how to drive a stick. These comments have come from other drivers of this truck as well. I know how to drive a stick, so it is not operator errors. The engine is not in need of a tune-up. In the winter time according to my friend the engine runs better. Thanks! AD
 
Top