1994 Mark viii idle issue

[BigUziLinc]

Hi everyone,

I have an issue with my mark viii where the car idles very rough at 500rpm where the whole car sways side to side and has a loping idle that goes up and down ~50rpm. Also on the first start the car will rev up to 2k rpm and then stall, the second start it will struggle but will manage to keep the car running until it warms up to operating temp and then will stall. Once I start it again and leave it by itself it’ll idle with the above mentioned issue.

Once the car is warm and I go WOT it rev hangs ~1000rpm and then it starts to idle normally and very smoothly at ~650rpm but after a minute there’s a thud and the idle drops to 500rpm and becomes very rough with the whole loping issue again.

This problem only started happening after having changed the IAC, maf, tps sensor, spark plugs, wires and coils with new ones. Prior to changing the parts the car would idle fine but a bit higher at around 650 rpm but would have a jerk of rpm and if I wasn’t pressing the brakes hard enough nudge the car forwards while idling at traffic lights.

I have replaced the spark plugs with new warranty ones and they are gapped to the factory spec, have thrusted with old coils and wires and the issue persists. I get no check engine lights but using a obd 1 scanner I get DTC that say lean mixture.

Sorry for the long write up and any help would be appreciated to help find what’s going on.

 

[xtriggerman]

the crank position sensor goes bad often enough on these cars and should be changed as well. if that doesn't help, I'd look into the secondary intake flaps to see if they are shut at idel and only opening over the 3K rpm mark. if they are sticking, it sounds like what you have. Way too much air at low idle. the flaps are a pain to fix from what i read but maybe some one can chime in on their exact name and how to get them working again.

 

[xtriggerman]

Here is what I was talking about. Those carboned up round flaps should only open once the RPM's hit 3000 and above that they should be peged at WOT. If they are stuck as some have reported, you can get what Biguz is describing. Thats why its all ways good to do occasional WOT runs just to free them up time to time. a good running car will give you a noticable shove forward when those secondaries open up. My car is spot on at 3K with that.

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[BigUziLinc]

Hmmm that is definitely a possibility but I’m not sure if it the secondary intake flaps because if I feather the throttle until 3k+ rpm then the issue persists. For some reason it’s only WOT that seems to fix the idle for a bit. I also didn’t have any issues prior to doing the “tune up” described.

I also think the cars getting too much air. So I am actually suspecting that the new iac valve is a defective unit. But I’m not sure if a bad iac would cause a low idle and also an irrational idle.

Here’s a link to a Reddit post of my issue with a video embedded in it:

 

[xtriggerman]

Try unhooking the MAF. If the car smoothens out, that could be your issue. Also, make sure your Vac lines are good. Mine had tiny cracks in them that messes up the AC / heat controler. You can test Vac lines with a "smoker" device. Did you try putting your old IAC back on?

 

[BigUziLinc]

Unhooking MAF will be the next step.

I don’t have a smoker but that is also the next step if maf doesn’t seem to be the issue.

As an update i did change the iac with a new replacement one. The car now starts without stalling the first time but the crank time is quite longggg. Also on cold start ups the car will still stumble and have a rough idle with an hesitation and delayin acceleration where the car almost does if I step on it a bit before jumping up in rpm until it’s somewhat warm and I press on it a little bit then the idle will become normal. If I drive the car in the city I will have no problems after that. If I go on a 30 min highway trip once I come to an idle after exiting the highway the idle issue will come back and only a WOT will fix it.

I also get a check engine light after 5-10 mins of driving with codes 172 and 176.

 

[BigUziLinc]

Unplugged the MAF and the car idles fine ~ 750 rpm in park.

With the MAF plugged the car jumps between 500 and 650rpm in park idle. Doing a slight blip of WOT where the revs stay under 2k makes the idle return to 750 and stay there for a few mins before going down and jumping or if I go into drive and then back into park it’ll start jumping

 

[BigUziLinc]

Replacing the maf fixed the issue but the CEL still comes on after 5-15 mins of driving so there is also a vac leak somewhere

 

[xtriggerman]

Pug off the VAC going into the fire wall to the AC/heat controller. The controllers have ALOT of O rings in them that can get dry and shrink enough to give you leaks. There is a U tube vid that shows you how to replace all those little O rings. I did that to no avail and ended up buying a used "Automatic climate control " dash unit since the head unit wasn't working right. Pluging it off should give you a heads up if its a control leak. My manifold vac lines were toast when I bought mine. What are the CEL codes?

 

[BigUziLinc]

I get CEL code 172. I’ll test the climate unit but when I listen to the engine it sounds as if there’s a leak coming from the intake manifold or rear end of manifold. But I’m not sure as it could just be the normal intake noise

 

[xtriggerman]

The Vac line is in the back of the manifold. When the lines crack, you cant see it until you bend them. P0172 is upstream oxygen sensor is bad. I dont remember if its the right side or left side one but one is an easy change while the other is a PITA to get at. Hope its the easy side.......

 

[BigUziLinc]

I had the 02s replaced at a shop a few months back because it looked way too PITA to do them. Is the vac line the one that connects to the throttle body?

 

[xtriggerman]

I dont remember exactly where it starts from on the manifold but just pull them off and inspect the lines, They do crack. Any hollow rubber line needs to be inspected. Alot of vids on U tube about P0172 codes. A little searching can give you alot of ideas to check.