I've run the diagnostic procedure on my car to try to figure out the uneven idling I'm having (not rough, but inconsistent RPM). The test has narrowed it down to the oxygen and coolant sensors and I've ruled out the coolent sensor as the sensor resistance is correct for the engine temp, and I get < 4 volts at the sensor wire at the ECM connector per the manual. However it's running in open loop in idle (steady dwell), and grounding the O2 sensor or choking the engine doesn't make a difference. I also get continuity on the O2 sensor wire and a good ground on the O2 ground, both measured at the ECM connector. According to the manual I've either got a bad connection to the coolant/O2 sensors (not likely) or my ECM is fried.

The odd thing tho is when I run the engine up to 3000RPM I do get a varying dwell- it's just not occuring at idle. Because of this I tend to think that the ECM is actually still good as it is responding to the O2 sensor's input.

Before I go run out and spend $100 on a rebuilt ECM, does anyone have any ideas as to what might cause this, or do I really have a bad ECM?

Thanks!


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Original owner - '84 T/A w/stock 305HO 5-speed

 

not very scientific way to trouble shot but for 20 dollars you can replace the o2 sensor.

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-=ICON MOTORSPORTS=-
1st & 3rd

 

True, but cheaper than $100. Plus the O2 sensor currently in there is ~9 years old and is way past it's 30,000 mile replacement. It certainly can't hurt to change it out at this point- from how the car behaves it acts like it's 'borderline' right now- doesn't work well with idle temperature exhaust, but when the engine revs up the temp goes up high enough to give the ECM what it would consider good readings....

 

Here's my take on this.

I order for the ECM to recognize and begin to use O2 sensor voltage readings, certain things must have ocurred.

The sensor has to have reached and maintained a temp of 600 degree F (although I read recently of 400 F being the minimum). I'm sticking with 600 though since it's the figure most often quoted.

Second, the O2 voltage has to have cycled past the 450 mv level a certain number of times. I've never seen the exact number of times this has to occur, but there must have been a minimum number of cross counts that the ECM recognizes.

Engine run time, manifold air temp and coolant temps play into this too, but I can't recall the specifics. I can look them up in one of my TPI manuals if you need that data.

From talking with a few guys who were having problems with getting into closed loop, the problem stemmed from either a contaminated/old O2 sensor or headers that keep the exhaust temp below the temp threshold at idle.

Since you connections are okay, I'd first try a new sensor. If the engine goes closed loop at road speed but drops out at idle, then it's cooling off and you'll need to either tune the engine to increase exhaust temps a bit or go to a heated O2 sensor.(Timing and/or fuel)

Hope this helps.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

Thanks, Jake - it does help. The engine's stock so I don't think I need a heated O2 sensor- I'm running a stock exhaust setup, too. But it sounds like I'm going in the right direction with changing the O2 sensor as it's pretty old. Even if the car sat only 2 minutes after having been run it starts up with the idle problem again as if it was stone cold. This leads me to think that the sensor needs to run at a higher minimum exhaust temperature than normal before it starts to output a reading meaningful to the ECM, and it doesn't get there 'till it's been driven a bit.

I'll be changing out the sensor tomorrow night (as soon as I can find my 3/8" to 1/2" adapter and I'll post what happens.

P.S. BTW I'm running a electronic Q-jet so I'm afraid the TPI stats may not help me- they may not even apply for all I know.... but thanks for offering...




[This message has been edited by David Trimble (edited November 12, 2000).]

 

Installed the O2 sensor today- no joy. Absolutely NO affect. Oh well, the sensor should've been replaced anyway...

So I ran the test again. This time when I checked the dwell on the mixture control solenoid it was running WAY below 10 degrees and was fixed- in fact it was so low I wasn't even sure at first if the dwell meter was working. I choked the engine per the diagnostic chart and the dwell reached (but didn't exceed) 50 degrees.

What confuses me here is the chart is a little vague as to what to do if the reading is AT 50 degrees (for those of you with factory 1984 manuals I'm looking at page 6E1-31, figure 22). I assume that I probably want the selection 'Dwell increases to over 50 degrees' as I interpret the other selection 'Dwell did not reach 50 degrees'
as applying to a reading significantly lower than 50 degrees (say 45 degrees or lower). At least I hope I'm right in this assumption as the tests below this selection covers things I've already gone over before (basically steps covered for testing a fixed dwell between 10 and 50 degrees, which as I said in my first post I've already done)

Does this sound like a valid assumption? The alternative ('dwell increases to 50 deg') would be more plausible as it suggests readjusting the carburetor, and I had my carburetor opened recently to replace the accelerator pump plunger. I didn't touch any of the solenoid adjustments when I did this but now I'm wondering if I somehow messed something up during this....

 

DT,

The E4ME can be a little sensitive. Just opening it and replacing the air horn gasket can cause a change. It's been several years since I have toyed with a controlled QuadraJet, but it sounds like the mixture is off. The idle and intermediate wells terminate at the top gasket, and the change in position from disassembling the carb may have moved the solenoid enough to warrant a mixture adjustment. If the carb is clean and the enrichment solenoid is operating, the only thing left is to adjust he mix. Try the solenoid adjustment first. If that doesn't allow enough adjustment, it's time to pop the idle mix caps off the base and start adjusting he screws.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Vader- that's what I was afraid of. I didn't know how sensitive the electronic Q-jet is to being just opened up and closed again. Apparently it's rather picky. I'll get a set of solenoid adjustment tools/gauges and just pull the carb and recheck the settings, as well as giving the carb body the once-over to make sure I didn't dislodge something from having it opened the first time that wound up clogging a passage.

Thanks- I'll post my findings this weekend when I get a chance to do this.

 

Another question- in the manual it shows how to check the solenoid plunger travel before opening up the carb for adjustment (in fact the manual says this should be done first before opening the carb up) Would checking this measurement suffice, or should I go ahead and open up the carb and do the complete adjustment on the solenoid plunger regardless?

 

Well I checked the solenoid plunger travel last night- it's within spec, so I'm beginning to think I'm going to have to either readjust the idle bleed valve and/or break open the seals on the idle mixture screws and reset those.

Just how difficult is it to remove the plugs on those idle mixture screws? The manual shows one way of doing it (cutting slots in the throttle body under the plugs then chipping out the resulting tab and plug), but I also noticed Thexton has a tool to remove the plugs. Has anyone removed these plugs before (with either method), and if so- just how hard is it to do?