Mysterious Idle Surge

[SpawnedX]

Car:

1999 Mitsubishi Eclipse GSX
5 Speed

Problem Description:

Problem started as minor fluctuation 800-1200 range. Problem escalated to a 800 to 1700 idle fluctuation. Then car began to have fast idle, sometimes over 2000 at operating temperature after moderate driving time.

What I Have Done:

Tonight I pulled my ISC motor from the car, checked the resistance and confirmed the ISC was bad. I replaced it with a brand new OEM ISC from Mitsubishi. I replaced the BISS screw, o-ring and BISS screw cap with brand new ones from Mitsubishi. I then did the proper way of adjusting the ISC by going into the Solus and adjusting the BISS, target range is 750 +/- 50 RPMs. I was unable to get it better than 950, may be because these cheap plastic screws that only Mitsubishi would use strip out and I couldn’t safely tighten it down some more (have another screw inbound to try with a larger screw driver). So I tried adjusting the throttle stop on the back of the throttle body, to no avail, that got me to right around 1000 RPMs. Called it a night since I had to close up and leave.

The Problem Worsens:

Pulled the car out of the garage, the moment I got on the throttle and then released it, fluctuation came back immediately upwards of 1500 RPMs. After the highway drive home, car didn’t want to drop below 2000 RPMs, sometimes saw upwards of 2500, and say I was at 3500 RPMs, the car did not want to drop to 2500, it took quite awhile to idle that down.

I am lost. I have no idea what else to check. No codes are thrown.

Topic

[dreamer2355]

Im going to assume you did a sweep test on the TPS and checked the STFT and LTFT for signs of vacuum leaks?

Did the Solus show you by chance what position in % the IACV is at when you are at KOEO?

I know of some Nissan’s if you replace the IACV, you have to turn the key on for 10 seconds, key off, then key on and start for the idle to reset after replacement.

Jacob here has been great at been able to paste FSM info so hopefully he will chime in soon if its required.

[pcmdjason]

I used to have a Dodge Stratus with random idle crazieness and it was a wire bundle that had worn through and randomly shorting.

[SpawnedX]

I’ve done a boost leak test, which I thought would cover the possibility of vacuum leaks as well, but I didn’t check the fuel trims was pressed for time (was at school and it was their Solus and the school was closing). From what I understood though, that since this is a MAF style car, if there was a leak after the MAF it would cause the opposite issue which is low idle.

And no on the ISC %, the way it works for this car is the Solus holds the ISC at a fixed position while we adjust.

[djdevon3]

I know nothing about Mitsubishi’s but if your car has a throttle position sensor maybe look into those readings as well as the MAF and O2 sensor readings.

[dreamer2355]

Any updates yet?

You don’t have a sticking throttle plate do you or a issue with the throttle cable?

[pcmdjason]

Do you know if you’re running a 4g63, 4g64 or 420A engine in your eclipse?
And is it turbo/Non-turbo?

EDIT: OK The GSX is 4g63-Turbo.

I’m still leaning towards a wire-short or a MAF issue.

You’ve got an OBDII system so try to get a cheap cable and pull some live data from it.

[EricTheCarGuy]

It does sound like a situation where your getting too much air at idle, it’s not a good idea to adjust the throttle stop however as this will offset the TPS and possibly not send a signal that the throttle is closed so make sure you get that back to where it was and that the throttle can close fully as I’ve seen throttles not adjusted correctly causing this very problem. BTW the TPS should read .5v or less at closed throttle and over 4.5V at WOT. If you don’t find a mechanical issue then you might consider resetting the system by disconnecting the negative battery cable and touching it to the positive for about 5 seconds, this should reset the computer and put things back to the base settings. Lastly you might want to check the operation of the PCV system as this can sometimes cause issues that act like vacuum leaks. Good luck and keep us posted.

[SpawnedX]

Quoted From EricTheCarGuy:

It does sound like a situation where your getting too much air at idle, it’s not a good idea to adjust the throttle stop however as this will offset the TPS and possibly not send a signal that the throttle is closed so make sure you get that back to where it was and that the throttle can close fully as I’ve seen throttles not adjusted correctly causing this very problem. BTW the TPS should read .5v or less at closed throttle and over 4.5V at WOT. If you don’t find a mechanical issue then you might consider resetting the system by disconnecting the negative battery cable and touching it to the positive for about 5 seconds, this should reset the computer and put things back to the base settings. Lastly you might want to check the operation of the PCV system as this can sometimes cause issues that act like vacuum leaks. Good luck and keep us posted.

Thanks Eric for the tid bits. I haven’t been able to get back here since posting (or even working on the car for that matter) and have a new symptom to report, the car is bogging down at 3000+ RPMs and, particularly under boost. I believe this indicates that I have a new leak in the system. From deduction I know it can only be at the throttle body where the upper intercooler pipe bolts on or at the coupler on the other end of it, since that is all I removed to replace the ISC. I ordered another BISS and o-ring, so I am going to go ahead and pull the throttle body again, pull the bottom off, clean the FIAV and replace the BISS and o-ring another time. From there I will be re-adjusting the stopper to factory specifications (which from what I am reading is to adjust the screw until it just touches the plate and then an additional 1 1/4 turns).

I believe my new issue is unrelated, and here is what I think is causing it.

The previous owner replaced the factory upper intercooler pipe with an aluminum one. Unlike the factory and some more quality kits, the flange portion to the throttle body is very thin. I believe that because of this, the head caused it to warp (based on a visual inspection as well) and it is no longer sealing against the throttle body properly.

My original issue I am leaning towards EGR or the FIAV is worn out. I really hope it isn’t the EGR because I planned on eliminating it after inspections, so I would like to not need to spend a $120.00 for one day of use, but it is what it is. I will also check the PCV system, I didn’t even think of that, goes to show you what experience is worth, thanks for that call. I do have a new PCV anyway, so one way or the other it is getting replaced.

Because this is a heated throttle body, I also have to consider my CTS. Since I finally worked a deal to go finish my certificate at school, I will be looking it over tomorrow. Hopefully with some positive results to report back

[dreamer2355]

Im looking forward to your updates. I doubt your EGR is the issue as it if was stuck open at idle, it would be running rough. If it was stuck close at highway speeds, it would run lean as oxygen would take place of the inert exhaust gasses.

[EricTheCarGuy]

I don’t see any benefit from eliminating EGR as it actually helps you run a higher boost without retarding ignition timing. I believe that is a MAF system which means that ANY leaks in the intake tube will cause performance issues so if you suspect problems there they should be addressed and then recheck for the problem. Keep us posted.

[dreamer2355]

Yup, alot of people seem to think removing the EGR is beneficial. The inert gas that is reticulated back into the engine is actually COOLER than the cylinder temperatures.

[SpawnedX]

Quoted From dreamer2355:

Yup, alot of people seem to think removing the EGR is beneficial. The inert gas that is reticulated back into the engine is actually COOLER than the cylinder temperatures.

I know there is no performance gain benefits from it, but there is the benefit of keeping the unburnt fuel from caking on your intake. Plus the EGR fumes are still much hotter than intercooled air (which a good intercooler should bring back down to around ambient). The main benefit for removing it, is removing a common component that fails.

I utilize ECMLink v3 in my car so timing isn’t an issue, I control all of the timing and fuel tables.

Right now I am thinking it is my FIAV. I have another TB on it’s way to me and am going to swap that in and see if it resolves the issue. I will let you know the outcome.

[EricTheCarGuy]

I think your way off base on that one as EGR really does benefit the engine and can help you run a higher boost because it helps prevent detonation and preignition under load, it also doesn’t gunk up the intake, that’s actually the PCV system, I’m not going to get into the specifics because I’ve done nothing but type for the past 4 hours but let me put it this way I don’t think EGR is causing your problem as I’ve never heard of anything like that happening because of an EGR problem. If you have an EGR issue it normally shows up as a surge on the highway or an occational miss going up a hill or under load. In a situation like yours you might want to check for an exhaust restriction as well as check your turbo and it’s controls as any faults there would most defiantly cause a power loss, leave the EGR and other engine controls alone as you may end up creating more problems by eliminating them, just because your ‘buddy’ did it to his car that doesn’t mean it was a good idea. I’m speaking metaphorically if you didn’t catch that. T)

[SpawnedX]

I’m only trying to resolve the maintenance aspect of things on the car. The EGR valve will be removed in the future for really unrelated purposes. The car is being built for track purpose and will be running e85 and tuned through what amounts to a full engine management system ( http://www.ecmtuning.com ) to avoid any issues with detonation or pulling timing. Our EVAP system is very large and goes underneath the engine and poses multiple vacuum/boost leak points to chase down. By removing the system, I allow myself significantly more room. I understand fully what everyone is saying about the EGR and I agree on a normal daily driven pump fuel powered vehicle it serves no purpose to remove it. Let’s move away from that for a second.

I am more concerned with the EGR as a source for a post-throttle body vacuum leak that is pulling in additional air. The Mitsubishi Service Manual and other resources list the EGR as an item of interest to check. I don’t want it to be the EGR, because I don’t want to replace something that I will be removing in the future. I also am not concentrating on that item right now. I am more convinced the problem is the FIAV which is why I have a new throttle body on the way (as well as need shaft seals, biss and biss o-ring and new throttle body gaskets) for me to rebuild. I just don’t have time to devote to the car to really give updates on the issue (only work on at as a project at school for credit right now).

Here is where I have gone for this in order and what my next step is:

– Boost and vacuum leak test (resolved some common small ones).
– Pulled the ISC and checked the coils, one was bad.
– Replaced the ISC, screws and o-ring for it.
– Replaced the BISS and it’s o-ring.
– Used the Solus to put the ISC in a fixed position and adjust the BISS (only able to idle it down to 1000 and fast idle and surge still exist).
– Tried to adjust the throttle stop to get it into range. No luck, so adjusted it back to stock specifications.
– Boost and vacuum leak after reinstallation of throttle body and intercooler piping.
– Checked CTS to ensure proper operation.
– Checked both O2 sensors.

Next up:

– Rebuild used throttle body with verified good FIAV, including new seals and gaskets.
– Obtained an original metal BISS rather than the new plastic ones Mitsu sells.
– Re-adjust TPS to factory specifications.
– Use Solus to set idle again.

Depending on those results, I will begin to look into the EGR as it is the last post throttle body possibility. The MAF checks out good, but I used a confirmed working one from my 98 GST just to be safe.

[EricTheCarGuy]

Keep us posted on your progress then, good luck.

[Author]

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