I have an intermittant code 33. 1988 automatic What also occurs but without setting a code is a high idle that will go hang at about 1200 to as high as 1500 rpm in neutral and about 800 to 900 rpm in drive.

I have done the basic tests for code 33. A friend let me use his Snap-on scan tool and it seemed to show a bad maf. Replaced it with a rebuilt and replaced the relays. No change. Used a Snap-on Vantage meter to check the wiring, no obvious issues. New iac and new tps. Any thoughts. Thanks for any useful advice.

Oh, yes I used the search feature and used Google to search the site which works pretty damn good just enter your search terms and then site:http://forums.corvetteforum.com/foru...0&daysprune=-1.

 

Do the extended test for the high votage at the MAF. Ask AGENT 86 for his pdf or do a search for it. You either have a bad connection at the MAF/ECM, a short in the wiring, or a bad MAF/ECM. Follow the troubleshooting in a FSM.

 

Thanks for the quick reply. I forgot to mention that I did some snooping around by the battery and found the ground leads in the corrugated sheath that was sitting on the sharp edge of the frame. A section of the sheath was missing so I put a piece of slit heater hose over it. Of course then the proble seemed solved but it wasn't.

 

http://members.shaw.ca/corvette86/Code%2033.pdf

 

Thanks. I probably have that link bookmarked. I am going to try a few more circuit tests and then run down to Kohlweiss in Redwood city to pick up a rebuilt ecm, $88.00.

 

Reman ecm and it seemed to run great but then threw a code. To make it more interesting, there was the added code 36. Could that have been a positive sign that things were starting to make sense? So I followed that troubleshooting path. Everything seems good. While I was checking continuity and voltages, I yanked on all the assorted wire bundles with no fluctuation in the numbers. The final say according to the service manual flowchart as I follow it is a faulty ecm. Hmm. Oh, yeah, when it is started after sitting for half an hour the idle is pretty rough and surging but it will stabilize after a minute or two.

 

Wish i could help you. I haven't had to deal with these codes so i can't offer much advice.
http://members.shaw.ca/corvette86/Code%2036.pdf

 

I had this problem a few months ago on my 88 4+3. I didn't have any codes however and it turned out to be my o2 sensor. Once I replaced the O2, the problem went away and hasn't returned since.....

JAT

 

So, there is a code 33 and a code 36 now? After replacing the ECM?

I would fix the codes first. Fixing them could solve the statment above. It could also be injector or O2 sensor related as well.

 

read paragraph two of code 33...pull the fuel pump relay off the firewall and check the wires going into it. You may be surprised at what you find.

 

I didn't see any issue with the wiring but your mysterious advice leaves me curious. Could you be making a reference to the oil pressure sensor intermittently dropping out?

While driving it today, I was aware of what felt like an increase in drag while coasting. I don't remember where I saw it but it was written that if the ecm goes into limp home mode, it will fell that way. I copped out and threw some more money at it on a wish that maybe my six month old rebuilt maf was bad. No change with another. I ran all the code 33 tests again with the car jacked up at the left front to see if perhaps body flex was causing something to be intermittent. All the numbers were the same. I did throw a new O2 sensor in it just because it has 97k miles and it looks like it had the original. I need to go drive it again and see what happens.

 

No I wasn't, and you can't see the problem (insulation shrinks back, exposing bare wires, in my case they were coming into contact with each other) without pulling it from the firewall and looking behind. What I was refering to was paragraph 2, "The oil pressure switch or the ECM, through control of the fuel pump relay, will provide 12 volts for the MAF power relay which provides the 12 volts needed by the MAF sensor."

 

No especial progress so far. I hooked a dvm up to terminal B12 on the ecm to monitor the maf voltage while driving to see if anything funny happens. I noticed that at 600 rpm idle in gear it was below .74 volts. When the idle would hang at 900 rpm in gear I would see about .90 volts. My thought then for the high idle is that there is air passing through the throttle body but not past the throttles which leads me to suspect the iac because the higher voltage means the air is being metered which would not happen with a vacuum leak. There is some slop in the throttle shaft but monkeying with it while the engine is running doesn't affect the idle i.e. snaping the throttle open doesn't cause the idle to increase or decrease from where it is hanging.

I did have it code a couple of times while I drove. Once or twice I think I managed to be looking at the dvm when it stumbled. This is at a steady cruise on the freeway with the cruise control on. The voltage I saw at the stumble was about 2.4 to 2.5 volts which at less than 1/4 throttle should set code 33. The fsm says that an open on circuit 998 which goes to B12 will pull the voltage high thus setting the fault also. So, I guess a simple test would be to run a new wire for circuit 998. I have the dash partially apart for other reasons so acess is pretty easy.

In fact, duh. The iac is probably getting heatsoaked and sticking because the high idle deal happens after the car has warmed up. What I ought to do is go out and whack it with something while the engine is still hot and see if there is any change.

 

Wouldn't it have been easier to have said that in the first place?

 

Cut him some slack, sometimes the only pleasure is to be cryptic on a forum.

 

I would double even triple check the MAF connection. I've never read anyone posting about heat soak on the IAC, the main problem encountered with the IAC is carbon build up..good cleaning takes care of that, a new IAC is about $30. and includes the elusive gasket. You might try spraying some carb cleaner around that TB shaft and see if the RPMs increase.

Didn't mean to be cryptic....just trying to help.

 

The succinct story for the high idle, tps. The problem stared me in the face and I ignored it.

In more detail, leaving out the f-bombs, feel free to include any as you read this. I did some brain thinking, read my Chevy fuel injection book and came up with an approach, throttle just the tps not the throttle shaft and read the maf voltage. Guess what? the maf voltage increases with an increase in the tps. If you read the previous post, I saw higher than normal idle maf voltage when the idle was hanging.

I had noticed that putting pressure on the tps with the engine running would affect the idle. Since I had installed a new Delphi tps less than a year ago I didn't think about it. The high idle seemed heat/time related. Watching the tps voltage as I drove and seeing it stick at .74 at a 1k rpm idle led me to World Famous Lee Autoparts while I was getting coffee in Alameda and I got a new tps, Standard products.

Will this cure the code 33? I don't know. Seems more like an intermittant on circuit 998 but I'll get back to you on that. Driving in traffic on 880 it didn't reoccur.

Thanks for all the advice.

 

If your post wasn't so darn cryptic I wouldn't have to ask for clarity! Did the new TPR stop the high idle or the IAC?

 

So far it is running great. The new tps seems to have done the job. Rock solid 600 rpm idle in gear with the brakes on.

 

Sounds good. I hope that you have this problem fixed.