So I recently had the car down for a week doing some routine maintenence while replacing 2 fuel injectors. I replaced all the vacuum tubes, cleaned the throttle body and IACV and air cut off valve, and replaced gaskets on the entire top end of the motor. I also painted a few parts that had corrosion and general wear and tear.

While doing all this I dissassembled the throttle body and removed the TPS, power valve, vacuum tube junction, upper and lower intake manifolds, EGR valve assembly, and took apart the IACV and all attached parts. I also drilled and tapped a hole for the aux temp sensor.

On a side note, the Engine Coolant Temp Sensor was VERY slightly damaged while it was removed to drill and tap that tube. The tip of it got a small gouge when it was dropped.

When I removed the IACV, the step motor retaining piece slid off and the valve inside was turned so the it moved outward... but I made sure to turn it back all the way in and put back the valve retainer as instructed in the FSM .

I then turned the screw on the ECU all the way clockwise and then all the way back counter-clockwise in order to reset it.

In my opinion, I reassembled the car perfectly and everything looked to be in the right spot. The distributor is exactly where it was before and the TPS is also. The injectors all ohm to 11.3 when cold and 12.5 when hot. I'm getting a spark from each wire. Clamping the blow-by tube does not affect idle speed so I don't think it's a vacuum leak. The MAF is also reading good on the volt meter.

The car started up totally fine and ran great for about a day. However, now the car idles very rough when it is cold and has a distinct misfire. This goes away when the car warms up. In my opinion, a slight gouge shouldn't change the resistance enough to greatly alter the cold A/F mixture.

What steps should I go through to diagnose and fix this misfire? Can a poorly installed injector cause a cold misfire? Is it the slight damage to the temp sensor? Did I turn the ECU the wrong way? Should it not be ALLL the way counter Clockwise?

The misfire will go away after 10 minutes of driving, but is pretty bad while it's there. Also, the car will chug under heavy loads such as driving in 5th gear at 1750rpm. This never used to happen. I just want it back to where it was before the injectors went out 2 weeks ago.

 

Sounds like a intake air leak somewhere, most likely at the intake plenum gaskets...

 

sounds like the opposite to me. unless the heat is closing the leak.
when cold the engine idles higher which helps to mask a vacuum leak until the engine warms up enough to lower the idle making the leak more apparent.

because the coolant sensor is so cheap, I would replace it anyhow just to rule it out.
I would examine where you painted, if any of it is on a mating surface, then i suggest removing the paint and smoothing out the surface to the correct contours.
other than that
if you are still in the area, feel free to bring it around and I'll take a quick look at it

 

Thats my theory...The same thing happened to a local persons 5th gen, idled like shat when cold, then drove fine after warm, ended up being an intake gasket leak.

 

stranger things have happened.

 

yea like my farking axle. i mean seriously.

i had my car throw a cts code at me last month, i swapped it, the symptoms didn't improve then, but soon thereafter after i let my car sit for 3 weeks while i waited for my other 3rd gen to come home and make a decision which one got the new clutch and which one got to keep its parts, i put it back together, and it hasn't misfired cold since. it runs like crap b/c of some failing injectors or something, but the cold misfire is gone as far as i can tell.

 

I'm sure there isn't a vacuum leak because I put propane from a torch around the entire perimiter of the intake and nothing changed. I'd never paint an important mating surface, so I'm ruling that out. For example, I always tape off where hoses connect to parts like on the EGR valve and vacuum tube junction. Paint has a nasty way of gumming up over time or making it impossible to remove hoses in the future.

Also, the fuel rail is dry and it I don't smell any either, so I'm pretty sure that the O-Rings and cushions are all seated properly.

Today when I left work after the car sat for 9 hours it started up fine, but by the time I got to the end of the parking lot began misfiring again. At a stop light I reached down and turned the screw on the ECU almost all the way clockwise, waited for the light to blink a few times (1-2??) and then turned it back a bit. The misfire went away for the rest of the ride home, however there is still a slight chug on high loads. I'm thinking I should focus on the ECU, but even after reading the FSM I'm not sure how to properly set it. I think I remember reading that the screw can control the A/F ratio.... can anyone chime in with ideas?

Brian, I'm working at Navistar in Melrose Park so shoot me a PM and let me know how close that is to you. If I end up needing too, maybe I can stop by during lunch if it's close enough.

 

alright, well I'm pretty sure I fixed it. I ran the diagnostic mode 3 on the ECU and found out that I had O2 and Knock sensor codes. The knock sensor is broken, but wouldn't be giving me the cold start issues. I ran diagnostic mode 4 and broght it into regular driving mode. I have a good feeling that the cold issues were a result of driving the car in one of the diagnostic modes, though I'm not sure which one.

Tomorrow morning will tell if my theory is correct. I'm hoping to GOD that it is....I just want my baby back to the way she was before this overhaul. The motor looks BEAUTIFUL after the clean up job. I'm sourcing a few clips and new distro cover, then I'll post some pics.

 

the ecu resets to mode 1 every time you shut the key off mang...

 

Like a hive of blondies - one is a red light jumpin snake ... now choose the most beautiful?

Cleanup? Was water incl? If so, TPS & Cap is now soaked?

Cap is a breathing institute: if u get drop lurking, breathing will suck the suckers in and cause moist to change spark routing. This would fit loading misfire... TPS on the other hand only used when cold.

And until u got rEAL WIRES, no way sparks will arrive to plugs as dreamed.

 

No my stery t here: I dont say this is the [only] explanation, but one issue is weather = ambient humidity - cap was dried.

 

I'm over near lincolnwood, not really a lunchtime jaunt, unless you take 2 hours for lunch...

 

Well I'm running Magnecor KV-85 high performance wires... They're very good IMO.

I didn't use water to clean the bay, so I don't think any got where it wasn't supposed to be. Either way it was like 5 days ago so any that was there would be dry by now.

I've come to the conclusion that I can stop the misfire by turning the screw on the ECU.... and it doesn't matter what mode I put it in, either. For example, this morning when it misfired I put turned the screw clockwise and waited for one blink then turned it back. This afternoon I waited for 5 blinks and turned it back. Both methods stopped the misfire. Anyone know why this would be??

The manual says to make sure all sensors are working before entering any diagnostic modes. The knock sensor is totally broken. I dont' think so, but could that be causing the cold misfire? Maybe just because it's a broken sensor the ECU is doing something to try and compensate while the engine is cold? I might have broken it while doing all the other stuff, but for all I know it's been broken for months.

On that note, if anyone has a spare working knock sensor for a 93 VG (don't know if they're year specific) then I'll need to buy it from you. I'm also posting a WTB thread for a knock sensor.

 

i've got a spare everything (except injectors and stuff that typically gets ruined in a tbelt breakage) for a VG. only code it threw was for the CAS, which it assumed wasn't working b/c the tbelt was broken. as for the screw fixing the misfire, the only thing turning the screw does aside from switching modes is to fully seal the iacv, and then reopen it once you turn it left again. so maybe crack open the iacv and verify that it's nice and clean in there?

 

so when the car is cold is the IACV open or closed? Do you think that the IACV is not opening/closing on it's own when the car starts, and has to be forcibly moved by turning the screw? Is that possible? I opened the IACV when I was polishing everything, so is it possible I damaged it or did something wrong when I turned the step motor? Would that have an affect on a misfire?

 

So said the ol gypsy salesman of his stolen ol horse teeth

Are they wires or carbon soot hoses? The latter requires magpies to carry the spark read http://www.cardomain.com/ride/748507/12

Has CAP been inspected with magnifier lens, bright light?

If turning a swicth changes electronics behavior --->the that sw is bad. Here ECU selector sw?

Try KS 5c bypass http://www.cardomain.com/ride/748507/9

IACV works always stepped by ECU ctrl. When one spark is missed, IACV is adjusted - so it follows - the error correction action. Badly idling engine - IACV stepper is in constant run opening and closing the valve: Listen, screwdriver on IACV, ear on handle... If it jams, can cause misfire difficulties to expand...

 

traxtar, was it a stumbly random misfire or a consistent rhythmic misfire?

 

Well, when it happens it's a consistent misfire. Although it seems like I can feel a slight misfire when driving at highway speeds... I could be mistaken though.

They are wires.... don't use carbon dust at all. I know it's not relevent but they're a $200 set of wires. That doesn't mean I paid that much, but you get my point. They're a VERY nice set.

Wiking, how many watts is the 500kohm resistor you used? It looks like 1/4 watt.
So is the general consesus that a bad IACV would cause a misfire while driving? Does it even have an effect while driving, or only the idle speed? I just don't know what is actually wrong, because turning the ECU screw will fix the problem... thats the part that confuses me.

 

I would change the Coolant temp sensor, everything you're describing my car did when mine went bad, installed a new one and everything went away... It's nice to rule it out even if it isn't too though, especially since you said it was damaged anyway.

 

Good!

I guess KS bypass is the 1/4W - doesnt really matter.

ECU screw is a switch. Bad contacts or lost contact with pcb - printed circuit board...? My guess the sw can be replaced quite easily after ECU is on the table. Just make sure problem always follows the play... Start from checking charge voltage.

IACV will do its work 'below 1000rpm' - when warm. Bad IACV is most asthmatic when coming to stop...

 

Well, I made some progress on the problem. Let me know what you think about maxpwr/my theory.

So I figured it wasn't an ECU or sensor problem, because it was intermittent and could be reset by turning the ECU screw. The only thing I could see the screw-turn doing was lowering the idle, turning on the diagnostic select mode, and closing/opening the IACV. So since the problem could be stopped with a simple screw turn, I started to check the things that the screw turn effects. The problem shouldn't be ECU related, because sometimes it works and sometimes it doesn't. In my experience electronics tend to experience go/no-go failure... either they work or they don't. This doesn't take a loose wire into account, but just bear with me. I figured the IACV was fine because I could feel it working, and the problem wasn't idle related. So I figured it must have something to do with the injectors.

I verified this by unplugging the injectors one by one, and noting the change in engine speed. Well wouldn't you know it, I unplugged the number six injector and plugged it back in and the misfire went away. The wires are not open, because they ohm at 0.05, which is normal for a short length of wire. So I don't think it's the connector, but the injector itself. Maybe the ECU IS the culprit of that kind of problem?? Thoughts??

I'm guessing the injector is getting stuck open or closed (probably closed), and unplugging it will cause it to start working again. Is there any way to fix or properly verify that my theory is correct? A light that plugs into the injector connector would verify if it's the ECU or not, but I can't make the problem happen and it seems like unplugging fixes the problem, so trying to put a light on when it starts misfiring wouldn't work. The light would have to be on BEFORE the misfire occurs, and that's impossible to predict.

 

hmm, well have you verified the +12 signal coming TO the injector? ecu controls grounds, remember. it kinda does seem like a loose wire of some sort, like unplugging and re-plugging it get the connector to fit up properly again and not have an opening wire.