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Searching for idle / surging

8K views 7 replies 5 participants last post by  Mazda Carnage 
#1 ·
hey guys im new to this forum been on pp and pt for a while however, but ive been asking around on both of those forums about my car searching for an idle it will warm up at at about 2k then after it warms it will search from 1500 to 2000 and back also ive noticed after its warmed up and been running for a while it wont want to start again untill its cooled off some. I know about the low coolant in these cars messing with the idle but thats not the problem ive tried burping the cooling system several times now with now change, ive also done a smoke test to see if the intake had any leaks and i found none if anyone has any information it would be much appreciated
 
#3 · (Edited)
That's gonna be the IAC for sure. I went through a LOT of trouble fixing this same problem.

I believe there's a guy on the forum selling another good IAC. Mine tested good electronically, but the mechanical seal was old and worn and letting air bypass/leak through. Sounds like yours is doing the same as mine did.

Also, go to the dealership and get a replacement gasket. It'll be about $30, but if you are friendly and ask nice enough, you can try to negotiate it to about $25 shipped. Definitely replace that gasket or you'll be back to chasing that leak.

I also made my own IAC hose from PVC pipe (I believe I used two 1.5" 90 degree bends and had to get a 12" of 1.5" pvc tubing, though you really only need like 1.5 inches). The old hose is probably worthless and getting a new hose is going to be way too expensive. The PVC piping cost me about $1.50, including gas to get to the store and back. I did use PVC cement to join the 3 PVC pieces together, and had some left over hoses that are used to connect the PVC to the inlets on both the throttle body and the IAC. Works perfectly. Be sure to use permatex or some form of gaskset sealer between the hose and the pvc. I had a leak the first time and it was annoying having to go back and "patch" up the homemade IAC line. Been running strong with no issues for almost 1,000 miles now.




There should be several working/good IAC's on ebay. You shouldn't have to pay a whole lot for a working one, maybe $40-$60 if you are lucky. There's a lot of "NEW!" ones going for $250 ish... The dealership doesn't have any available in the US (I checked), and even if they did they're listed at over $450.

You can try to "clean" your IAC, but I'm going to go out on a limb and
say that the internal seal is old and gone. You can't take it apart to repair it. Save yourself a lot of install/uninstall/reinstall headache and just find one that you know works and replace.


Oh, you are also going to need a new throttle body paper gasket. Those are super cheap. Good time to clean the throttle body out...
 
#4 ·
Is it necessary to fix it? I ask because mine does something similar. Mine will idle up to 2k and then do the 2 stage thermo drop(drop to 1300 then again to 900 in my car) and every once and awhile back to 1300 again if its a cold day and im on the interstate for a bit. I think the overall syatem is working fine as far as idleing high for warmup and then a two stage drop-howerver isnt 2k a little high for warmup idle speed. Makes me wonder if my iac is leaking pre warmup- is this possible in the realm of anything is possible-hmm. So I ask does your ever drop idle speed or does it always search: ie after warmup does it idle fine?
 
#5 · (Edited)
2k warmup? Depends on how cold it is outside I guess. I suppose mine will start at 2k, but it'll drop to about 1300 real quick, unless it's about 40*F outside. But I don't let it idle for more than 10 seconds or so (especially in cold weather) before taking off nice and easy. The car warms up faster if you drive it gently and load the motor, driving helps keep oil pressure up, quicker you get it up to operating temp the more efficiently it's running, etc. Some people disagree with this, and that's okay, this is just my philosophy.

I think you mean "surge" when you say "search"

I've been keeping a close eye on my car for a while now. Its been 250 miles since i replaced my 02 sensors and cat, and my car is in Ready status and there's no pending codes. So I think my emissions and CEL woes are behind me. It's been a warm day today (~75*F high), and I noticed at one point my car was idling at around 950rpm with no load. It did this twice as I was in traffic and its the first to happen though I've set it to idle at 650rpm (+/- 25rpm). But no, once warm, it should just idle at 600-700rpm smoothly unless something loads the motor, like the AC compressor engaging/disengaging, thermostat opening and closing and radiator fans turning on/off, etc.

Necessary fix? Depends. My car would shut off after a while of driving and have issues starting up unless I let it sit for about 20 minutes. Its dangerous to have a car just shut off on you while you're driving. Get it fixed.

Can you bypass it? sure. You'll have a running car, but it won't be running efficiently; it'll run rich. You'll possibly ruin your spark plugs, foul your 02 sensors, ruin your cat, and wasting gas. It'll drive and do so as dependable as if it were running, with a few caveats.

You can block the IAC. To do this. Just take the IAC hose off and put bypass caps on the throttlebody Inlet and the IAC outlet. You don't even need to unplug the IAC or remove it with a block off plate, and this will save you trouble from having to reroute the coolant.

You'll probably have to adjust your idle a little higher. Factory is 600-700 RPM, I had to set mine to 800-900 RPM to get it to idle smoothly. It'll still bounce a little, maybe 100 rpm deviation or so. Also, when you lift off throttle or have a sudden drop of RPM (like throwing the car in neutral and letting the rpm drop from 3k to idle), the engine will definitely drop below your set idle, so you got to make sure your idle is high enough that the motor can catch this quick enough and not stall out. With the AC on, my car would stall out every now and then, but I had no trouble with the AC off. If I was going to keep this setup, I'd have to raise my idle to about 1000-1100 to make sure I didn't stall out with the AC on. The IAC helps detect load on the motor (ie: AC engagement, etc) and will bypass air to help keep the motor from bogging down and stalling out. Without it, you've got to manually compensate by raising idle RPM.

Word of caution though. I bypassed my IAC with bypass caps for about a month deciding what to do with the car. This will cause your car to run rich since the IAC is not functioning properly (air metered by VAC isn't agreeing with O2 sensor output, not enough air). The ECU sends a PWM (signal) to the IAC telling it how much to open, thus letting extra air into the intake manifold. Knowing there's this extra air going into the manifold, it adds more fuel. Since you're IAC isn't really bypassing more air through, you're going to run rich. This will foul plugs and the O2 sensors upstream in the manifold, and probably ruin your catalytic converter. Also, if you have an OBD II car ('96,'97), it'll throw an IAC CEL after about 50 miles of driving. I'm not sure if a CEL will be thrown on OBDI ('93-'95) cars, as people removing their IAC altogether didn't have a CEL. After fixing my IAC issue, I had to replace my O2 sensors and my cat had shown signs of overheating from burning rich. My MPGS droped to 22-23mpg when I typically get 25-26.

So if you have to pass emissions upon your safety inspection, you're gonna need this IAC working (or apply for an emissions waiver).
 
#6 ·
The BAC/ISC isn't always the cause of surging or searching idle.
Whats happening is to much air is getting to the engine at idle (in high vacuum) how that air is getting there is the cause of the high idle, the dropping and raising (surging or searching) is the ISC and air valve trying to regulate the idle back down to its proper setting but the amount of idle to control is out of the ISC's range so it shuts completely and the idle drops and opens a bit and the idle climbs and this repeats itself over and over, so you look like an idiot revving the engine over and over again at every set of lights.
The problem can be throttle body related or a sticking Vane air flow sensor (Vaf) or a gasket or vacuum line after the TB or the Bac/Isc gasket leaking or the Bac staying opened after the engine is warm.




To test the ISC/BAC:
Unplug the two wire isc/bac plug with the engine idling cold and note the RPM, then do the same with the engine at operating temperatures. Check that the engine speed lowers and is lower then with the valve disconnected and the engine cold.

The resistance at the ISC/BAC plug between the two wires withe the engine warm should be 6.3-9.9 Ohms (if not it's defective).

I know in very cold weather the N/A F2's could experience this due to frost build up after the TB, the 90+ models run coolant through that spot of the dynamic chamber. (GT and N/A got the coolant changes on 90+ models but I have only seen it happen on non turbo engines in extremely cold weather, the F2T creates more engine bay heat and compressing the intake air warms it despite the intercooler or perhaps the longer plumbing and it's angle allow less condensation to get into the throttle body?).
 
#8 ·
Porterprobe does it turn over normally when it's hot and wont start?
-If so: It could be the fuel pressure regulator solenoid control valve, it sits on the firewall
and intercepts the fuel pressure regulators vacuum/boost source hose. The water thermo switch, the vaf or the throttle sensor.
-If not: it could be the solenoid on the starter going out and when it gets hot it wont work.
Do the battery poles get really hot when the car wont start? (careful can burn skin)
This would indicate a bad or weak connection at the battery terminal or at the large transmission ground or at the 12v main + going to the starter or the 12v starter wire going to the solenoid. or corrosion in either of those wires.
 
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