Code 21: TPS test procedure help. Paging Gerry!

[22229stingray81]

After taking a break from working on the ol' vette, I'm back at it again. It has been throwing an intermittent code 21 (TPS). I have cleared the codes and it didn't come immediatly back on, but it has done this a couple of times now.

I'm looking at the test procedure in the shop manual (page 6E1-20), and I have a very stupid question. First thing listed is "Ground 'test' term." Does this mean jump the D and E terminals on the ALDL connector under the ash tray, or something else? Thanks.


-Tatortot

 

[Hib Halverson]

You got it!

Ground the diagnostic pin of your ALDL to the chassis then turn the key on and count the flashes of the check engine light.

The DTC 21....if you've cleared codes then start the engine, does the light stay off for a given period of time, then come on and stay on until you clear codes, again? Does that behavior repeat? That is, you can clear codes and not see the light for a while but then it comes on, again?

 

[22229stingray81]

Thanks for the reply, Hib. The code 21 check engine light usually comes on shortly after kicking off the fast idle after a cold start. It will flash once, then go out, and the code will be stored. It has done this off and on for quite a while now. Yesterday I disconnected the battery for several minutes, then let it idle in my driveway for 10 or so minutes, and the light did NOT come back on, but I feel confident it eventually will as it has in the past.

Just to be clear, I have attached the test procedure from the manual. In the top square it says "Ground 'test' term." So this means I should perform the outlined test while the computer is giving me the codes, correct? Thanks again.

-Tatortot

You got it!

Ground the diagnostic pin of your ALDL to the chassis then turn the key on and count the flashes of the check engine light.

The DTC 21....if you've cleared codes then start the engine, does the light stay off for a given period of time, then come on and stay on until you clear codes, again? Does that behavior repeat? That is, you can clear codes and not see the light for a while but then it comes on, again?

 

[Hib Halverson]

Thanks for the reply, Hib. The code 21 check engine light usually comes on shortly after kicking off the fast idle after a cold start. It will flash once, then go out, and the code will be stored. It has done this off and on for quite a while now. Yesterday I disconnected the battery for several minutes, then let it idle in my driveway for 10 or so minutes, and the light did NOT come back on, but I feel confident it eventually will as it has in the past.

Just to be clear, I have attached the test procedure from the manual. In the top square it says "Ground 'test' term." So this means I should perform the outlined test while the computer is giving me the codes, correct? Thanks again.

-Tatortot

Follow that procedure. Do it as soon as the engine light comes on. Shut off the engine, set up your dwell meter and DMM the way the instructions say then turn the key on with the engine off then take the measurements.

Does this only happen on cold starts with the carb on the fast idle cam and not after a restart when the engine is at operating temperature?

 

[22229stingray81]

I haven't noticed it any other time besides after a cold start, HOWEVER, I drive the vette so little that I couldn't tell you if it comes on after a restart. I generally drive it around the block, 5-10 miles, once a week or so. I will start paying closer attention to when I see it.

.... sorry if I'm being dense, but have we confirmed that I jump the ALDL for the attached test, or is there some other terminal I need to jump? It states that I need to start the engine up and I've never done so with the test terminals jumped. No danger of shorting anything out? Thanks.

-Tatortot

 

[Hib Halverson]

I haven't noticed it any other time besides after a cold start, HOWEVER, I drive the vette so little that I couldn't tell you if it comes on after a restart. I generally drive it around the block, 5-10 miles, once a week or so. I will start paying closer attention to when I see it.

.... sorry if I'm being dense, but have we confirmed that I jump the ALDL for the attached test, or is there some other terminal I need to jump? It states that I need to start the engine up and I've never done so with the test terminals jumped. No danger of shorting anything out? Thanks.

-Tatortot

Your not being dense. Questions are good.
The manual states to turn the ignition on but do not start the engine. No danger of shorting because you're connecting the test terminal to ground.

 

[22229stingray81]

Roger that, Hib. I was looking farther down the line when it said to start the engine, but I'll cross that bridge when I get to it. I'll report back when I perform the test procedure. Thanks!

-Tatortot

Your not being dense. Questions are good.
The manual states to turn the ignition on but do not start the engine. No danger of shorting because you're connecting the test terminal to ground.

 

[KANE]

In my experience... temperature changes do weird things to electrical components that are on their way out. I had a TPS earlier last year that operated fine when cold but once the engine was up to temperature, it started to misbehave and send weird values.

Parts and connections will malfunction when cold or hot- but usually not both with normal in between. And if just didn't work period, you'd find it quickly. So, you'll get readings that are normal until they get hot or abnormal until they get hot. I think it's the nature of metal and contraction/expansion that has to do with shorting out.

Regardless- it sounds like your problem is something that is related to something that is on its way out but not so bad that you can easily find it. Those are the most annoying things. :W

I'm curious to know what it the culprit was as well!

 

[22229stingray81]

Thanks for the info, Kane! It's got to be something weird like that... I've been fighting a surging/rough idle issue and gas mileage problem for a couple years now. Did extensive vacuum tests and fixed a lot of little stuff. Gas mileage is up to 12mpg, which is acceptable, but idle still surges a bit, and this intermittent code 21 keeps coming up. Gotta wait a few days for the temps to come up to do the TPS testing. I'll keep reporting back, with a bunch of noob questions, I'm sure :L

Speaking of which, is there a way to test the TPS plunger for good movement WITHOUT taking the carb apart?

-Tatortot

 

[22229stingray81]

Okay, the CEL has not come back on since I posted. I also performed the first box in the test procedure. A to C was 5.2v, B to C was 4.8v, so that checks out as good.

Next test procedure says to start engine and let it idle. Does it matter if it is warmed-up or not? Fast idle on or off? Still have the test terminals jumpered? BTW, I filled up last week and I'm down at about 9.9mpg again. Ugh. ad

-Tatortot

 

[Peer81]

I would say, let it idle when hot around 650 rpm. When cold your dwell meter will point to the open loop circuit setting that isn't what your looking for.

Greetings Peter

 

[22229stingray81]

Makes sense, thanks Peter! Any thoughts on whether the terminals should still be jumped?

-Tatortot

I would say, let it idle when hot around 650 rpm. When cold your dwell meter will point to the open loop circuit setting that isn't what your looking for.

Greetings Peter

 

[Peer81]

Jumper is when running and see what happens. Keep us updates

Greetings Peter

 

[22229stingray81]

Thanks, and will do!

-Tatortot

Jumper is when running and see what happens. Keep us updates

Greetings Peter

 

[22229stingray81]

Okay, I went for a 15 mile ride today, then completed the test procedure. Dwell was around 24 degrees, so I moved along. Resistance between A and B was 31kΩ, and A and C was 27kΩ. So it looks like I need a new TPS sensor ad

I'll probably start a new thread, but does anybody have any tips to replace it? I need those "special" tools to adjust it, right? And do I need to take the carb off?? Thanks.


-Tatortot

 

[GerryLP]

Wow. Sorry for the hugely late replay

Tatortot,

I will send you my phone # over PM so that you can text me when you have an issue.

Here are a couple of old images I had posted before.

The troubleshooting chart is talking about the TPS connectors A,B, and C.

 

[22229stingray81]

Thanks Gerry!

For those following along, I've decided to partially rebuild the carb. New TPS and accelerator pump for sure, and maybe even more if I'm feeling particularly brave. I'll report back when the work is done.

-Tatortot

 

[22229stingray81]

Okay, I got the new TPS in, and it has been adjust to .56v. Also a new accelerator pump, and air horn gasket. It doesn't seem to have cured my problem, but the rough idle seems slightly better. I haven't done another fuel consumption test yet.

I decided to keep looking for other vacuum leaks. I tested both front and rear choke pull offs, and the rear one is no longer working; it cannot hold vacuum. I've been searching for a replacement, but apparently the L-81 didn't come with a choke pull off? Can you L-81 owners confirm your carb does not have a rear choke pull off? Why would some one add one? Perhaps this isn't the stock carb?

The vacuum line coming off the rear pull off goes to a Tee connector. One side is going in to the base of the carb at the rear, and the other has a very small hose or cable going into it that disappears into the firewall somewhere. I can get pictures if need be. The pic below is the only pic I had of the rear vacuum break handy.

 

[Antz81]

I decided to keep looking for other vacuum leaks. I tested both front and rear choke pull offs, and the rear one is no longer working; it cannot hold vacuum. I've been searching for a replacement, but apparently the L-81 didn't come with a choke pull off? Can you L-81 owners confirm your carb does not have a rear choke pull off? Why would some one add one? Perhaps this isn't the stock carb?

No Rear pull off on mine.
I did think that your carb number looked a little odd (17081222). Nowhere i look has this listed as a carb for an 81 vette. What does your date code on it say? I'm thinking its not the stock carb.

 

[22229stingray81]

Thanks Antz81; it's all FINALLY starting to make sense. It should be 17081228 for a non-CA automatic Corvette. Apparently 17081222 is off of 5.0L automatic Malibu, Impala, Caprice, El Camino/Caballaro, Camaro, Century, LeMans. UGH. I wonder what the differences are? And I wonder how much horsepower I've lost. Probably not much.

Well, now we know I don't need the rear choke pull off. I have removed it, and the tee connector, and now I have that single cable/wire going straight into the back of the carb. Could someone tell me if this is remotely correct? And what the hell does that little cable go to? It terminates under a little metal box to the right of the wiper motor behind a relay. Also, I cannot remove the linkage without taking the choke housing completely off... so it's just hanging there for the time being.

Before removal:

After:

Small cable terminates: