I recall that a few weeks before my TPS crapped out, the engine began idling rough. Most noticable in closed-loop.
I replaced the Oxygen sensor. Made no difference.
Several weeks later, after replacing the TPS and accelerator pump washer, hot idle was still rough. Acceleration is wonderful, and there is no roughness above 800 rpm. And for once, no warm-up bogs. AND, the car now cold-starts with two blips of the gas plus a short cranking. Used to take several cranks and much pumping.

When I pulled the plugs, they were spotless and odorless, so I assumed that we are running lean. WRONGO!!!!!!!

Because the exhaust has been smelling, I then assumed cat issues, so I pulled the cat today, and put in an exchange unit.

Still smells- BUT ONLY IN CLOSED-LOOP, SEEMINGLY.

Question: the mixture solenoid does work, but could it have worn out? There is no raw gas smell on the plugs, though, and the cat has not exploded. Remember, I did the most minor of rebuilds on the carb. Also, I thought that the computer would flash a rich code.

EDIT: Just changed the H.T. wires. Idles better, but 'no cigar', as far as the idle being clean enough to not smell.
Wierd, though that THE FAST IDLE IN OPENED-LOOP IS ODOR-FREE, BUT SLOW-IDLE IN CLOSED-LOOP SMELLS.

and, TO TOP THINGS OFF, WHY DOES MY P.C.V.. VALVE MAKE A CONSTANT BUZZ????????, THAT I HAVE NEVER NOTICED BEFORE?
Am I onto something, here?

I apologize for all the 'upper-case' letters.

Thank You,

Seth

 

Is it the computer, or has the mixture solenoid worn out?
Idle smells in closed-loop, not in opened-loop.
I hate to be a repetative pest, but.......

Seth

 

NoTransistors,
I've seen several of your posts and I gather you are probably more "Technically Smart" about some of these things than I am... But I'm going to ask a potentially stupid question...

Have you verified 30 degree dwell on the MCS in closed loop?
Do you have the .48 volts on the TPS?

I think you and I were both looking for some help with a Code 21 a couple weeks ago? Mine still throws a 21 but not nearly as frequently as it had in the past. The only change I made was to adjust the Mixture Screws (leaner -1/4 turn in). I now have a MCS dwell of roughly 35 degrees and the code 21's are basically gone.

If I understand the principles here, and I know I'm not near 100% with this yet, I think asking the MCS to stay on (dwell) slightly longer...


Even if I don't understand it 100%.... it doesn't matter ... It's running much better at low RPM

P.S. I NEVER "smell" fuel with this vehicle. When I was younger I could tune my '78 Trans Am just by sniffing it's tail pipe.

Kurt

 

Kurt,

I know so much about cars, but none of that knowledge extends to computerized engine management.
The dwell on the mixture solenoid (untouched when I pulled the carb), is perfect.
The voltage on the replacement TPS is .27 volts, as I totally f**ked up my access to the adjustment screw, BUT, if the idle is raised from 550 to 650, the TPS voltage is appropriate.
Computer shows no codes, and except for a rich closed-loop idle that must be delt with, the car runs nice, starts nice, and has some *****.
Now, if the car were a vintage Hi-Fi, it would be playing records, by now. That technology, I can deal with.
So, the mystery is, what to do?
Perhaps I should get that 'special' inspection, and in the spring, spring for a non-computerized Mallory plus a new Holley or Jet-rebuilt Rochester.
Would be wonderful to salvage this system, though.

Seth

 

I had an "expensive/special" inspection last year on this Z-28.


Fortunately, in Georgia this Z-28 ('82) is considered an antique/collectible AFTER next year's inspection and is completely past any/all inspection requirements.

I will pull my hair out but this car will STAY stock GM even though it's more difficult!

Kurt

 

Kurt
Kurt
Kurt,

Remember that I had ruined my chances of accessing the TPS adjust-screw? The remnants of the plug are clogging the bolt-tip.
I figured that the setting was close enough, after struggling longer with it, than with the entire rebuild.
Today, between the raindrops, I took a different approach to the TPS adjustment.
There is a rod linking the accelerator linkage with the rocker arm that works both the pump and the TPS.
What I did was, to remove that rocker, bend up the tip of the link rod, reassemble and test the voltage.
Did this until I got a setting of 0.48 Volts. This certainly is close enough.
I lost about 1/16" of pump travel, but the exhaust has lost that stink.

AN EDIT: Went for Chinese takeout. Car still runs nice, but---idle stinks again. Now, what did I do? Instead of raising the TPS, I lowered the arm. Simple, right? Have to check the voltage tomorrow. I am pissed.

BTW, my special inspection is $75.

Have a good one.

Seth

 

Seth,
I had done the same basic thing. I got the cover plug out just fine but didn't have the 2mm female to make the adjustment. So I went and screwed around with a tiny drill bit and tried to cut a slot in the top of the screw.....needless to say it didn't work.

I ended up buying a junk carb from the J.Yard for $5 and robbing the screw from it. I finally got smart and purchased a carb adjustment kit from skyway tools for about $12. Autozone and Advance don't seem to have any REAL tools. NAPA was out-of-stock. I got to check my spares....I may have a good CCC-carb top. I'll let you kow if I do......

INFO PROVIDED FOR ANYONE THAT NEEDS CARB TOOLS....
http://skywaytools.com/
Skyway has a minimum order of $20 so I ended up getting a couple other cheap tools I didn't really need to make-up the $20 rather than get charged for nothing.

KURT

EDIT: Just read that post again...
Wouldn't you also be adjusting the travel of the plunger. The TPS adjustment screw only moves that tiny little hook looking arm that contacts the corner of the TPS. If you adjusted the linkage plunger linkage you would be adjusting both. I believe the idea is to get the TPS into the correct position (up or down) by using the screw. This gives the ECM knowledge of the location of the Plunger and it will then know when to enable (or disable) the MCS.

When you wack the gas to WOT (or to a significant opening) doesn't the ECM determine it is time to take a back seat and go into OPEN loop?

If you are artificially moving the lever (hence both items Plunger + TPS) to get the TPS voltage to the proper setting wouldn't that mis-inform the ECM?

Maybe I'm all wet and don't know what the heck I'm talking about (or asking) but moving the linkage doesn't seem to be the right track?

Kurt

 

Kurt,

What a wonderful gesture.
I hope that it works out.
Whatever it costs, I am grateful.

Thank You

Seth

EDIT: Kurt- You might be right about this. Considering the fact that things were working for a bit; I will do the repair the right way, after receiving another carburator top. And in addition, if you have that rod that I fooled with as well, wonderful also.

Thank You,

Seth

 

Seth,

I have the top shown below. I do in fact have the arm... That's the easy one.

The top I have (one that is NOT damaged) is for what I think they call a 2-Point system? Somebody please help here!!

Basically, this one (shown below) does not have a RICH STOP mixture adjustment port for the MCS. It has a port to access the LEAN STOP and of course the AIR BLEED but not RICH STOP. This top is basically stripped and will need most of your parts. I do however have the lever.

If you have the "Two-Point" (that may be the wrong term) you are in bigtime luck! Other wise you're not because the other top I have is damaged.

KURT

 

Another photo...

 

Kurt,

Looks like that blank spot can be drilled out for rich-stop adjustment?????????
This is only if the thing is otherwise the same.
I will check my carb now. Back in ten.

Nice pics. Nice TPS screw access.

Seth

 

NO. I don't think that would work. The RICH stop is threaded into the top from the underside... Some of the Rochesters are like this one. I think I grabbed this piece just for the good TPS screw. The rods & hangers, air bleed valve, and other small junk was removed.

If this doesn't fit the bill... I can look for one for you at the local J.Yard. Would be very easy to find and probably very cheap but I cannot go to that particular J.Yard for three weeks. They are only open until 2:00pm on Saturday and I have Baseball games with my youngest son the next two Saturday mornings, not near enough time to get there and return. Most everything is Closed on Sundays (Atlanta is must be the center of the bible belt).

Kurt

 

Kurt,

Before we find the need to look further-

That little tower with your arrow, has a blanking plug in the center, with a deep hole in each of the two outer holes.

Furthermore, the area just rear of this has a rivited-in brass-colored cap.

Same carb?

Seth

 

Additonal Photo....

 

Kurt,

Nice photo, but my carb is on the car, so I cannot use it for reference.

So far, we might have a match, but what do I know about this carburator?

Seth

 

I Think you are in BIG-TIME luck!

The brass cover has to be removed to adjust the AIR BLEED. All of the Rochesters have that cover. No one I know of ever puts it back on.

If you have the BLANKING PLUG in location "A" I suspect it is not actually a blanking plug but the EXACT SAME no RICH STOP top as that would be where your RICH adjustment would be if you had one.

Directly in front of that "PLUGGED" hole should be a "Stand-Alone" opening "B" that is for the LEAN STOP you should be able to see down that hole and see the special screw.

Take another look and let me know..... I have to send a Wonder Bar I found out to my good buddy SellmanB tomorrow and I could send both if this is the right thing

 

Kurt,

Deep holes are in location you have red arrows pointed to.
I will now check rest.

Seth

 

I'm checking for a way to determine (FOR SURE) if you have a four point or a two-point.

Kurt

 

Kurt,

Please see my email. And let me know what the 2 point/ 4 point thing is.

Seth

 

You have mail!!!! Please check it now.
Kurt

 

You have a 4-point adjustment. 99.99% of them do. You have both a rich and lean stop plus the TPS and IAB adjustments. Trust me on this one.

MC Solenoid going bad? It can. The only "test" you can do on it, besides noting if it clicks is to check it's resistance across the 2 terminals. It should read ~20 ohms if it's in excellent condition. Spec is anything over 10 ohms. When they get old the resistance can go down and cause problems even though it will still "click".

"Starting point" adjustments for your QJet's various settings (do in order):

1. Lean stop. 4 turns up from lightly seated.

2. Rich stop. Adjust until there is exactly 1/8" of total mixture plunger travel. You can measure this through the little hole where the IAB valve screws in. Remove the IAB, stick a small screwdriver down there until it sets on top of the "home plate" that is the top of the solenoid's plunger. Use the top of the airhorn tower as a guide and make marks on the shaft of the screwdriver in both the "up" position and the "down" position and compare the distance between the two marks with a machinist's ruler. MAKE SURE TO DO THIS CHECK WITH AN AIRHORN GASKET IN PLACE AND WITH THE AIRHORN SCREWED DOWN TO THE MAIN BODY OF THE CARB JUST LIKE IT WILL BE WHEN INSTALLED. Gasket thickness and compression WILL affect this distance.

3. TPS. You know the deal with that already. Most of them read real low at idle (~.30V being typical). Doesn't matter. Close enough is good enough- the ECM is a real dumb bird in these cars. Anywhere reasonably close is fine.

4. IAB (Idle Air Bypass). 4 turns up from lightly seated.

5. Idle Mixture Screws. 4 turns out from lightly seated for a stock application. 5 turns out if it's a mildly modified engine or a bigger engine than stock.

That's how I do all my cc-QJets. From there you can do some tweaking if you like but I find they are pretty close to perfect 99% of the time without any further adjustment.

 

Damon,

Kurt is sending me a carb top without a rich-stop adjustment hole.
The rest of the piece looks like a match.
If this is so, I need to make a small tapped hole for the adjustment screw (there is a mark in the casting that will place the hole.
After installation of item on carb, I will adjust to your suggested baseline specs.

And, my nose tells me that a dramatic improvement in emissions seems to occur when the TPS is adjusted properly.

Thanks

 

Sorry to butt in, but I'm curious...when you said the dwell was perfect, did it vary with change in idle mixture screws and/or air bleed adjustment? I've had one of these peg at 50% at idle and no adjustments would change it. Could even crank down the air bleed until the engine quit. A swapped O2 sensor and readjusting rich/lean stops brought it into line. Never figured out what the problem was and why the ecm cycled at 50.

From what I've gathered (and I haven't yet found a sequence of operation for the ecm) the TPS should only tell the ecm when to stop managing the air/fuel mixture (WOT) and when the throttle is steady for torque converter lock.

Let us know how it goes.

 

I'm in agreement with NAF. If you're in closed loop (where the ECM is actually looking at the O2) it really doesn't matter what the throttle position is- the ECM is still trying to hit 14.7:1. When it gets up over about +4V it goes into WOT mode and specs a very rich (fixed) dwell scheme for the mixture control solenoid.

Remember you MUST check dwell in closed loop or you will always get "perfect" readings that never change. This must be done by warming up the engine on the road for a few miles and then patching into the MC solenoid wiring without actually disconnecting it from the solenoid (or using the factory dwell check terminal built into the wiring harness- a single green connector that goes to the + side of your dwell meter while the - side of your meter goes to a good engine ground).

You'd need someone who could actually go in and look at the code in the chip for you to be sure what I just said about the TPS is true (I don't know the programming) but that's how it seems to act on all the cc-QJet systems I've worked on.

Also- if your carb is currently a 4-point adjustment type carb then why modify the airhorn from a 2-point adjustment carb to work with it? I've got piles of old QJet airhorns laying around with 4 point adjustment. So does your local junkyard. Go get one if you need it- there're a dime a dozen.

 

Damon-have you observed two open loop modes? I know at WOT throttle the mixture solenoid goes full rich but I always assumed it ran full rich during warm up as well. If the ecm maintains a steady 50% during warmup that would make some sense. The choke should be richening the mixture already.

If that's the case, a bad O2 sensor would prevent the ecm from going to closed loop (in my prior case).

I've never watched the dwell with the engine cold before. I'll have to check it this weekend.

If someone has the code that would answer alot.

 

The way that you describe the TPS, it seems that the engineers would have been able to get by with a plinger-switch, rather than a variable resistor.
Timing does dramatically affect exhaust fumes, because......
When the TPS reads .1 or .2 volts, the exhaust stinks.
When I fool it into producing 1/2 volt, the stink goes away.
This is New York City. Most of the junk yards have vanished, and the few remaining ones are dirty, over-priced garbage dumps.
If the 2-point top simply needs the one limit screw installed, no biggie, if it needs more, then I will ask the kind people here to dig one out of their tool-box.
This replacement top also has an accessable TPS screw.
My O2 sensor is new.
I did the jury-rig on the TPS adjustment with the car running and the temp guage was then indicating 200 degrees. Besides, it is when the thing goes into closed-loop, that the exhaust smells.
I did not delve into other things, when I did the dwell test on the mixture solenoid. Did that in closed-loop too, I hope.
This morning, I had the car inspected. The deluxe-type inspection. Had little choice, as it was due in one week.
I still want to get the exhaust right.

Seth

 

Not arguing, you've brought up some questions that are interesting. I have read that the ecm looks at the TPS to determine that the throttle is constant before allowing converter lockup. That was in an electronic emissions control manual for computer carbs. As far as I know it's only other function is to indicate wide open throttle. A SOO for the ecm sure would be nice.

I think this weekend I'll break out the dwell meter and see what changes in TPS and CTS do to the dwell response. I really think the TPS only affects closed or open loop though.

How did your dwell respond to changes in TPS voltage?

Keep us updated. If there's anything I can help with let me know.

Mike

 

Mike,

I would need to investigate, to answer your question.
Also, your offer to help is very generous.
If the weekend is not a wash-out, and the parcel arrives, I may tinker.

Seth

 

NAF- yes, basically the ECM specs a fixed (but not full-rich) MC solenoid dwell during warm up. I don't recall if it's 50% or not but it's close and it's definitely fixed. You can fiddle with the IAB and the mixture screws all you want but it won't change during warm up. Coolant temp must be over about 100*F and the O2 sensor has to be up to temp (signal voltage flicking back and forth fast enough that the ECM knows it's gone "active") to go into closed loop. Then it starts to look at the O2 and adjust mixture based on it's readings.

A bad O2 might keep it out of closed loop but most likely it would simply try to correct the mixture to match the bad readings and end up making it run lousy.

I don't know all the things the TPS might affect but I suspect it's more than just one or two obvious things. Certainly TCC lockup is one, WOT mode is another but I think there's probably more going on in the code than just those two things. You'd need someone who has actual knowlege of the coding in the chip to tell you for sure, though.

 

When I replaced the intake gaskets last year, I needed to set the timing, as the distributor was out during the repair. Set it to 6 degrees, but would like to know what the smudged label really says.
Pigtail was unplugged, and, as everyone knows, that timing tab is hard to decypher; just plain wierd-looking. I hear 6 degrees; I hear TDC. What is factory spec on my model?

I ask because the base timing does affect idle emissions, and I am checking all angles.

EDIT: It is still raining.

Seth

 

TDC (zero degrees) for LG4, 6 degrees BTDC for L69.

On the timing tab, the large notch "valley" is TDC, the first peak towards the passenger side of the car is 4 degreed BTDC, every mark after that is another 2 degrees - 1st mark will be a notch valley, next a peak, then valley, etc.

 

Thanks.

Seth

 

83 LG4 are 6 degrees....

 

One thing about these cars seems to be a constant - If you think you know something that is written is stone, the factory is sure to have done it differently somewhere, somehow, sometime, for some reason. . .

 

Rebuilt the carb in fall. Replaced computer's temp sensor, O2 sensor, wires, ignition module, (no knock sensor to replace), plugs, even the cat.

Need to find the correct map sensor (who knows, this may be the problem).

Engine warms up producing CLEAN exhaust.
In closed loop, idle smells like that on my girlfriend's MINT '70 Mustang.

It has been too cold to tinker, and, as I have said, it may be the MAP sensor. A computer problem ?
MAP sensor problem? Owner problem?

I can see the map sensor showing the computer low vacuum, which would indicate a load that requires more fuel. Does logic work in this instance?

The carb cannot all of a sudden go rich. All of the adjustments are still factory.

Thank You,

Seth

 

Computer controlled QJet engines don't have a MAP sensor. They have it's evil twin- the VAC sensor. Looks the same, functions differently. MAPs go up when a VAC goes down (voltage). Don't play mix-n-match with junkyard parts. Get the exact right one.

 

Seth