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1G Rough Idle issues on warm startup.

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XC92

Proven Member
1,570
357
Jul 22, 2020
Queens, New_York
I'm having high idle issues too, although not quite as high, and in my case it's cycling from 1200~1600rpm. I've taken a cursory look-see under the hood but it's just too cold here to do a proper diagnosis and it'll probably have to wait another couple of months unless we have a warm spell.

I imagine that this is fairly common in cars this old. Mine's a '92, Turbo. When I do figure it out I'll post here in case it helps you or others. Hopefully nothing serious and one of the most common causes.

Stupid question, but does this tend to reduce mpg, especially if you drive on city streets a lot with stop signs, lights, traffic, etc.? I'm getting around 150-160 miles a tank, which is pretty bad, especially at today's prices.
 
I recall cleaning the throttle body and idle control on a different car, a 2010 Kia Rio, a while back, when it was having idling issues. This helped a bit, but what really helped was replacing the PCV and EGR valves. Would either of these cause idling issues on a DSM?

Also, on my Talon, I've also noticed startup issues when the engine was warm, i.e. after driving long enough to bring the engine to normal temp, parking the car to do some shopping, then trying to start it up again and having the engine run rough and die unless I pressed on the gas pedal.

Would this also have anything to do with such high or cycling rpm idling issues and what would cause it?
 
What really helped was replacing the PCV and EGR valves. Would either of these cause idling issues on a DSM?

Both of these when bad can permit air to enter the intake in quantities larger than normal and create idling issues. That's why they are on the list of things to check when dealing with Idle Surge.
 
Also, on my Talon, I've also noticed startup issues when the engine was warm, i.e. after driving long enough to bring the engine to normal temp, parking the car to do some shopping, then trying to start it up again and having the engine run rough and die unless I pressed on the gas pedal.
When I've had this problem -- twice, I think, on different Mitsus -- it turned out to be a leaking injector. It worked okay when the car was driving but when fully warmed up and stopped (like to do some shopping) it would fire, stumble, for several seconds before really coming to life. The solution was a set of refurb injectors.

Additional symptom noticed at that time: It was a little slow to start -- maybe cranked a couple extra revolutions -- when parked overnight. AND when after parking overnight I pulled off the fuel pressure regulator instead of getting a gush of fuel there was just some dribbling out -- the pressure was completely gone overnight.

The second symptom alone means a fuel leak 'anywhere.' But when combined with 'hard starting when hot' that acts like flooded (some cylinders fire, some don't at first) the first thing to check is injectors.
 
When I've had this problem -- twice, I think, on different Mitsus -- it turned out to be a leaking injector. It worked okay when the car was driving but when fully warmed up and stopped (like to do some shopping) it would fire, stumble, for several seconds before really coming to life. The solution was a set of refurb injectors.

Additional symptom noticed at that time: It was a little slow to start -- maybe cranked a couple extra revolutions -- when parked overnight. AND when after parking overnight I pulled off the fuel pressure regulator instead of getting a gush of fuel there was just some dribbling out -- the pressure was completely gone overnight.

The second symptom alone means a fuel leak 'anywhere.' But when combined with 'hard starting when hot' that acts like flooded (some cylinders fire, some don't at first) the first thing to check is injectors.
Was there a telltale leaking gas smell? I'm getting none at all. I'll look into this possibility but I suspect that it's a vacuum leak or faulty sensor or connector. I'm actually hoping to drop the fuel tank this spring to clean out the rust and gunk and will take the opportunity to check out the rest of the fuel system and fix or replace what's needed. Certainly the filter.
 
Was there a telltale leaking gas smell? I'm getting none at all. I'll look into this possibility but I suspect that it's a vacuum leak or faulty sensor or connector. I'm actually hoping to drop the fuel tank this spring to clean out the rust and gunk and will take the opportunity to check out the rest of the fuel system and fix or replace what's needed. Certainly the filter.
In your case it could be something like a leaking fuel pressure regulator, pull the vacuum line off of the regulator and check for fuel in the vacuum line, do you have a fuel pressure gauge on your car? it would be nice to see the pressure and if its leaking down between starts.
 
In your case it could be something like a leaking fuel pressure regulator, pull the vacuum line off of the regulator and check for fuel in the vacuum line, do you have a fuel pressure gauge on your car? it would be nice to see the pressure and if its leaking down between starts.
If the car didn't come with such a gauge then it doesn't have one. Engines and everything directly related to them, e.g. fuel delivery and injection, turbo, air intake, sensors, ECU, etc., are still mostly a mystery to me and I've yet to really study up on them. I've had so much else to do to get the car back in shape that I've put it off till now, e.g. seized brakes, really rusty underbody, bad clutch, bad trans, busted struts, old TB, etc. I still have a few other things to do but once I get to this issue, I'll thoroughly research and diagnose it. It's non-critical enough that I've been able to put it off so far. Getting the car reliably and safely moving and the engine not self-destructing too priority over a flaky idle and poor gas mileage.
 
Was there a telltale leaking gas smell? I'm getting none at all.
That telltale smell tells you specifically that you have a leak to the outside world. If the leak is an injector there won't be a smell except perhaps right after starting from the exhaust due to the extreme richness.

I'll bet if you used a bore scope down into each spark plug hole before starting after sitting overnight you'd see a wet cylinder if that injector is leaking. I wish I'd thought to try this when I was looking for that problem a while back. Those things are very cheap -- a tiny webcam with lights on it and a flexible stalk.

I've never seen it but it's conceivable that the FPR could leak either into the vacuum system or back to the fuel tank. The latter would be caused by rust on the valve plate and/or seat and given the number of 'long neglect' problems you're dealing with I don't think it could be ruled out. However this would cause a slight delay in starting after sitting quite a while -- not a stumbling start after stopping at the drugstore. (= 'flooded' as in the old days of carbs boiling over.) A fuel pressure test would confirm a leak if you just watch what happens when you switch off -- if the system doesn't hold pressure for hours it must be leaking somewhere. It will tell you nothing with the engine running because the fuel pump can keep ahead of anything but a gushing leak -- if you had that, you'd know it.
 
That telltale smell tells you specifically that you have a leak to the outside world. If the leak is an injector there won't be a smell except perhaps right after starting from the exhaust due to the extreme richness.

I'll bet if you used a bore scope down into each spark plug hole before starting after sitting overnight you'd see a wet cylinder if that injector is leaking. I wish I'd thought to try this when I was looking for that problem a while back. Those things are very cheap -- a tiny webcam with lights on it and a flexible stalk.

I've never seen it but it's conceivable that the FPR could leak either into the vacuum system or back to the fuel tank. The latter would be caused by rust on the valve plate and/or seat and given the number of 'long neglect' problems you're dealing with I don't think it could be ruled out. However this would cause a slight delay in starting after sitting quite a while -- not a stumbling start after stopping at the drugstore. (= 'flooded' as in the old days of carbs boiling over.) A fuel pressure test would confirm a leak if you just watch what happens when you switch off -- if the system doesn't hold pressure for hours it must be leaking somewhere. It will tell you nothing with the engine running because the fuel pump can keep ahead of anything but a gushing leak -- if you had that, you'd know it.

I see that a little differently. Definitely don't just wade in and change it to see what happens. But if an ohmmeter measurement or scan software says that the idle position switch isn't closed or reliably closed when the throttle is closed that car will never idle right until the problem is fixed. It can't because this switch tells the ECU to take control of idle speed and as explained, 'idle speed' is a complicated job -- engine warmup, air temp, accessory loads, dashpot (slows the rate at which RPM falls when you come to a stop so the engine doesn't stall) -- all are taken into account and symptoms can be just about anything that can happen when the throttle is closed.

The reason for the FSM caution is that it deals almost entirely with situations that arise in cars still being serviced by dealers and thus relatively low miles. There aren't a lot of us who own such cars now: Wear beyond what a dealer mechanic would have had to worry about in 1998 is quite common and the throttle valve assembly's not an exception. If the switch doesn't close reliably then the first step is to adjust it. If you can't get an adjustment that works then the switch itself is defective -- it can happen -- and the assembly needs to be replaced.

I haven't had very good luck with the FSM procedure using a feeler gauge, I think because there's wear causing the switch to behave a bit irregularly. What has worked better is trial and error until it closed reliably under all situations and opened immediately when the throttle did.
Thanks. Lots to think about and look into.

When I first undertook my overall restoration a year and a half ago I knew that after I was done with all the things I had to do to get the car running again and not likely to self-destruct once it was, e.g. unseizing all 4 calipers, replacing a shot clutch, fixing a faulty trans, rebuilding the struts, replace the way too old old TB, etc., the car would still have lots of lesser issues that I'd have to deal with because they either took away from enjoying the car on a day to day basis, like a barely working sound system, stuck power windows, dented body panels, etc., or were deferred maintenance gremlins that might be mostly annoying today but could turn into something more serious tomorrow, like worn out ball joints and connectors, old pads and rotors that still worked but not as well as they should, or this, a high and fluctuating idle that needs to be dealt with sooner rather than later.

I'll definitely get to it, along with the dozen or so other gremlins, annoyances and bugs that still need to be dealt with. But I'm pretty methodical so first I'll research it as thoroughly as I can and then plunge into it. Engines and everything feeding into them are still mostly a huge black mystery box to me. I get the basic idea of how they work, but the specifics still elude me. And it's the specifics that get you.
 
... but I suspect that it's a vacuum leak or faulty sensor or connector. I'm actually hoping to drop the fuel tank this spring to clean out the rust and gunk and will take the opportunity to check out the rest of the fuel system and fix or replace what's needed. Certainly the filter.
Best technique I know of for finding vacuum leaks: Spray a can of 'starter fluid' around the the flange where the throttle body joins the plenum then work down toward the engine. That flange is a common problem because people put the TB back without a gasket or with the old gasket or something is warped. Leaks there cause a smooth but excessive idle RPM where leaks at or nearer the head can cause a rough idle.

I wouldn't plan on dropping the fuel tank yet. It's a pretty big job, not worth doing without clear evidence of need. Pull the fuel pump -- it's under the back seat cushions -- and inspect the mesh strainer. If it's not solid brown glop you probably don't have a fuel tank issue. Look down into the tank with a strong flashlight: more than a haze is suspicious. Use one of those hose-and-bulb fuel siphon rigs to get a cupful of gas right out of the bottom of the tank under the pump: If it's clean and the bottom of the tank is shiny you don't have a problem.

If the tank passes inspection put a new strainer on the pump and put it back together.

The fuel filter won't cause an idle problem if the car runs at all on the highway. It should be replaced in a car of unknown history -- pretty much all of 'em now! -- but get through the problems that are right in front of your face -- the empty fluid reservoirs, the busted sensors and wires -- or that smack you with a real symptom like that 'idle speed varies widely.'

What I found helpful was to make a list of things that are wrong and whenever time allows, pick one there's time for today and tackle it. 74 items on it now, maybe half fully fixed in four months, less than one a week being added now.

In cars of this age there are very few 'just this problem' problems. Mostly when you start to fix this, you'll notice three more thats. I wanted to change the front bumper cover, got the new one ready and the old one off and discovered that most of the pieces of plastic ductwork and metal plates as well as many fasteners were gone and one headlight was way loose. Also the plumbing to the auto transmission oil cooler is just waving around and there's a leak by the oil filter. I've had that bumper cover off twice more since then, adding the newly found missing parts and there's still at least one more round.
 
Best technique I know of for finding vacuum leaks: Spray a can of 'starter fluid' around the the flange where the throttle body joins the plenum then work down toward the engine. That flange is a common problem because people put the TB back without a gasket or with the old gasket or something is warped. Leaks there cause a smooth but excessive idle RPM where leaks at or nearer the head can cause a rough idle.

I wouldn't plan on dropping the fuel tank yet. It's a pretty big job, not worth doing without clear evidence of need. Pull the fuel pump -- it's under the back seat cushions -- and inspect the mesh strainer. If it's not solid brown glop you probably don't have a fuel tank issue. Look down into the tank with a strong flashlight: more than a haze is suspicious. Use one of those hose-and-bulb fuel siphon rigs to get a cupful of gas right out of the bottom of the tank under the pump: If it's clean and the bottom of the tank is shiny you don't have a problem.

If the tank passes inspection put a new strainer on the pump and put it back together.

The fuel filter won't cause an idle problem if the car runs at all on the highway. It should be replaced in a car of unknown history -- pretty much all of 'em now! -- but get through the problems that are right in front of your face -- the empty fluid reservoirs, the busted sensors and wires -- or that smack you with a real symptom like that 'idle speed varies widely.'

What I found helpful was to make a list of things that are wrong and whenever time allows, pick one there's time for today and tackle it. 74 items on it now, maybe half fully fixed in four months, less than one a week being added now.

In cars of this age there are very few 'just this problem' problems. Mostly when you start to fix this, you'll notice three more thats. I wanted to change the front bumper cover, got the new one ready and the old one off and discovered that most of the pieces of plastic ductwork and metal plates as well as many fasteners were gone and one headlight was way loose. Also the plumbing to the auto transmission oil cooler is just waving around and there's a leak by the oil filter. I've had that bumper cover off twice more since then, adding the newly found missing parts and there's still at least one more round.
Thanks but I think you're mixing some of my issues with those of the OP. Perhaps it's best if you address their issues or else we're going sideways with mine. But I will take all this into account when I get to my issues.
 
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