Yup, I have it hanging out for now. Figured I might need to get to it again.

 

Ok, here is a quick rundown of everything I have done so far...

Fuel pressure:

38 @ Start Up
40 @ Shut Off
36 After 5 Min
32 After 5 More Min

Injector Ohm Test:

1=17 / 5=16.6
2=16.6 / 6=16.7
3=16.6 / 7=16.7
4=16.8 / 8=16.9

TPS Voltage: 5 Volts

A+B Voltage: 3.9 - 4.0 Volts

FPR: No fuel in Vacuum Hose of Fuel Pressure Regulator

Any Ideas? Other things to check? I am going to do a compression check later today. I am also going to do a Vacuum check.

 

The voltage measurement across the top 2 wires of your TPS should be .56 volts or so.....your injectors and fuel pressure are fine. If you can't get your ECM into diagnostic mode, I would look into that first. You should at least get a code that says it is o.k.

 

He is not really checking the TPS adjustment. I asked him to check the 5 Volt reference at the TPS because it was an easy place to check and make sure that the 5 VDC on the ECM is properly regulated and it appears to be.

The 3.9 to 4 Volts on Pin B along with the fact that he can't draw codes is probably the most important clue we have so far.

krynick, I will try not to forget when I get home, but please pardon me if I do. Once I get home and in the throws of action there, I may very well forget.

Hopefully someone will come along here with intimate electronic knowledge of this system OR maybe we will find a PCB schematic.

Stay tuned,
Doc

I started searching for an ECM circuit board schematic. If I can see the schematic of the ECM itself, then I can determine some checks that might lead us to the problem of why he can't draw codes.

I will be home in a while and I will look at the wiring diagram closer to see if there is anything else that might effect the diag mode. The first thing I will do is determine what pin on the ECM card is feeding pin B on the ALDL. By checking the voltage there, we can see if maybe there is a problem somewhere between the connector and the ALDL.

 

krynick,

Someone posted a link to some schematic information but it isn't what I expected to see.

Before you make the measurement I'm asking, did you get a DVM with high input impedance? If it is several Meg Ohm input impedance then go ahead and measure the voltage at pin A9 on the ECM connector to ground. If it is closer to 5 volts than to 4 Volts at that point, remeasure at Pin B of the ALDL. If the ALDL pin 5 is still 4 Volts then you have a resistive connection between pin A9 of the ECM and pin B of the ALDL connector. This could prevent the system from going into diagnostic mode.

If there is a difference in the voltage check as described above, then cross check by turning the ignition off and using your ohm meter to measure resistance between ECM A9 and ALDL pin 5. If there is resistance there, find it and repair it, but don't do anything crazy like soldering on the slide connections or anything.

Okay, here's another check for you. This will check the SES light output from the ECM: Check voltage at A5 referenced to ground. This should be 1Volt with key on, engine off and Battery Voltage with engine running.

BTW, you are checking the ALDL voltages with key on engine off (KOEO) aren't you?

If we can ever get it to go into diagnostic mode we might get somewhere besides feeling like we're shooting in the dark.

I sure wish you had an ECM to plug in and try!

I will check back in the morning. I'm usually at work before 5:30 so bedtime comes early for me.

Good luck,
Doc

 

MBDiagMan - Thanks for all of the great info. I am not going to be able to get to this until either Thurs or this weekend. I found a thread where someone had a very similar problem to mine including the mpg gauge. He replaced the ECM and problem was gone. I wish I had and extra ECM too, I went ahead and ordered a remaned one from Autozone today. I should have it by this weekend. I am going to still do the check's you suggested, maybe I can find a different problem along the way.

 

OUTSTANDING! I think there's a good possibility that the ECM will fix it, but I did not want to suggest that you lay out money for one unless I was convinced it was the culprit.

You will have to transfer the PROM chip from the old to the new. If you never removed and replaced chips from a socket before, be VERY careful. After you've done it awhile you can do it with confidence using nothing more than a small screwdriver, but if you've never done it before you might want to find the proper tools.

Failing a chip puller and installer, use a small screwdriver and pry in very small steps all around the chip. Raise it VERY slightly at one corner, then VERY slightly at the opposite corner, then the other two. Then pry it up just a little more all around and keep going in SMALL steps so that you don't pull up one end much further than the other, or one corner much farther than the other. If you mess up you can bend or break a pin on the chip.

When installing it, you have to start the pins on one side and then push the chip across to get the other side started. Until you get the feel of doing this you can indeed ruin a chip.

Good luck and let us know the outcome.

Doc

 

Thanks Doc. It will probably be in Thurs or Fri. I'll update you then.

 

Well, remanned ECM is now installed. Car seems to run beautifully now. It still smells a little rich, but that may be all of the excess from when it was dumping fuel in. I took it for a short run, but it is pouring out so I couldn't get on it too badly and I couldn't tell if it was leaving a cloud of smoke anymore. The MPG gauge works perfectly now and guess what??? The 4+3 is now working perfectly!!! I had never driven one (and didn't know how they worked) so I wasn't sure if it was working or not. When I installed the ECM the car was idling at 2600 RPM. The morons who sold it extended the little torque screw that controls the throttle position to hide the ECM problems. I backed it off until I was idling at 800 RPM's How do I go about adjusting that so It is correct?

My plans tomorrow are to Seafoam the car, then change the oil again, change the Trans fluid, gasket, and filter. Take it for a nice long drive and see how she is feeling.

 

Hello. Welcome to the forum. Glad to hear your car is running well. I am trying to find the information setting the minimum throttle plate stop and TPS. It is a bit of a process and I forget all the nitty gritty info. Now that you have a multimeter it should be no problem. If I have to I will scan in my printout from this forum from a few years ago. I thought there used to be a how to section in the stickies up top of this forum.

Some things you probably want to do would be clean the throttle body, and maybe the radiator. These cars vacuum junk off the road and deposit it in between the a/c condenser and the rad. There is no way to see it w/o removing the cover under the airbox. Yours may not be too bad, as you have low miles, but if it is hotter than you like, check it out.

Also, after running that rich for that long, I would not be surprised to see your cat go in the next few weeks once you start driving it. I had a similar situation when I bought my 85 (bad MAF). You will know if your car starts going really SLOW.

Dave

 

Thanks Dave, that would be helpful. I am going to google it to see if I can find something online tonight. It is nice to have the car running well now. If only it would stop raining in the north east so I can drive it that would be great!

 

OUTSTANDING! I felt that the ECM would fix the problem, but was probably being too cautious in making the call since it was not my money being spent.

To adjust the throttle plate, simply back it off until the throttle is closed and then open JUST enough so that the throttle plate doesn't touch the bore so that it doesn't stick. The computer and Idle Air Control motor will take it from there.

Black85 is right about the cat. Depending on how long it was running rich, it might be toast. If it does need replacing, you can get a high flow cat for less money than a stock replacement. There are many sources, but you might start by looking in Mid America or Ecklers catalog although theirs are overpriced. Also, if the cat is toast, then the spark plugs probably are toast as well.

I'm really excited that you saved this low mileage car from someone giving up on it.

Now about the 4+3. I have been driving mine for about 15 years and I have found that people either love them or hate them. I am a lover of this system, but I probably drive it differently than most people. Here's how I do it, if you want to give it a try:

I start up the car and once I put the car in low gear, by habit, I press the O/D button, toggling the O/D OFF. I then run through the gears and once I get up to speed, I hit the button to allow it to drop into overdrive. If I have to stop at a sign or a light, when I push the lever in neutral or low, I press the button again to turn the O/D off so that I am ready for normal driving. If I'm just scooting around town, I just leave the O/D toggled off.

My car is an 88 and has the OD button in the top of the stick so I can easily operate it with my thumb. I think yours has the button on the console, so it's probably not as convenient.

I believe that most of the people that develop a dislike for the 4+3 are those who just leave the overdrive to be operated by the computer. That can be very frustrating.

GREAT WORK ON SAVING THIS DARLING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Doc

 

Doc, black85, and anybody else who had input into my rich running problem I can't thank you enough. After Seafoam and another oil change the car is running perfect! I am averaging 21-23 mpg with a little spirited driving mixed in here and there. You guys were all huge help! Now I have to figure out my e-brake and I will be all set. Thanks again guys,

 

I'm still trying to find the specific procedure to set your tps and idle speed for the '85. I will just have to scan in the info if the tech tips at the top of the forum don't start working soon. It is basically the info from the 3 sections of the FSM that tell how to do it. Also, there is a tip regarding making sure your tps is adjusted so you reach open loop (4.5 v) at WOT (wide open throttle). This is important for optimum performance.

 

OK. I just typed it up. I needed the practice, and it was good to review the procedure. Hope this helps.

How to Adjust your Early C4 TPS and Idle Speed
By Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO
This tech paper will discuss the procedure for correct adjustment of the Minimum Idle Speed and for adjustment of the Throttle Position Switch (TPS) on the early C4 Corvette TPI systems. These steps apply specifically to the 1985 model year, and it general to other years. Later models do not have adjustable TPS's.
General:
Idle speed and off-idle response on the early TPI systems is determined by correct adjustment of the minimum idle speed screw combined with a correct setting of the TPS. I've seen many of these cars that have had their idle speed "corrected" by well-intentioned mechanice and owners by simply screwing the minimum idle speed screw in a few turns. This really messes up the settings, and will not make your car perform properly. Doing a correct setup of the TPS is one of the easiest ways make your car feel and respond better. To maximize the benefit of this procedure, I recommend that you first remove your Throttle Body (TB), disassemble it (it's incredible easy - there are a total of about 5 pieces in it...), clean the TB up really good with some spray carb cleaner, and put it back together. A nice clean TB will really put an edge on the performance improvement you will get by doing this procedure.
The Service Manual has instructions for doing these operations, but the directions are scattered through several sections of the Manual. Here is the complete, step-by-step process for doing this (not including TB rebuild). All specs and steps are taken directly from the Manual (all 3 different sections), and this process is absolutely correct.
Tools and Equipment
You will need the following tools and equipment:
1. A set of Torx wrenches. You can buy a complete set in a nice, genuine plastic pouch at Sears
2. A good digital voltmeter that will read voltages less than 1 volt
3. A paper clip
4. A small screwdriver

Procedure
There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC. You will also be playing with the diagnostic connector under the dash. Remove the cover (if it’s still in place). Bend your paper clip into a “u” shape. You will be playing with the two top right hand terminals (“A” to “B”) in the connector.
1. First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It’s located on the driver’s side of the TB. Remove this plug if it’s there.
2. With the IAC connected and the ignition “OFF,” stick the paper clip into the diagnostic connector from “A” to “B.” This grounds the diagnostic lead.
3. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds.
4. Now, with the ignition still in the “ON” position, disconnect the IAC connector at the IAC.
5. Remove the paper clip from the diagnostic connector.
6. Start the engine and allow it to reach normal operation temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while.
7. If your car is an automatic, set the parking brake and put the transmission in “DRIVE.” If your car is a manual, leave it in neutral.
8. Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
9. Shut off the engine and reconnect the IAC.
That’s it for idle speed. Now on to the TPS.
There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Min America), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.
1. Turn the ignition to the “ON” position without starting the engine.
2. Loosen the TPS Torx adjustment screws.
3. Set your volt meter to the low scale DC volt setting that will accurately read less than 1 volt.
4. Measure the voltage between the two top TPS wires.
5. Adjust the TPS by rotating its position until you get a reading of .54 volts.
6. Tighten the Torx screws and recheck the voltage. Readjust if necessary to make sure voltage is right at .54
7. Turn the ignition “OFF.”

Addition by CorvetteForum Member Charles Warner

As an addendum to “How to adjust your early C4 TPS and idle speed,” I would like to add that checking your WOT TPS voltage is also, potentially, a worthwhile endeavor. Frequently the TV cable that controls shifting pressure to the transmission binds the throttle linkage thereby not allowing for WOT. After measuring and, if necessary, adjustiong the idle voltage (nominally .54 volts) check the WOT voltage and ensure that approx. 4.5 volts is realized. A minimum of 4 volts at WOT are required for the ECM to go into fuel enrichment mode (FEM). If you are not seeing over 4 volts at WOT you are probably not opening the throttle to the stops and are not seeing FEM. A significant performance improvement is possible with this adjustment.
Charles Warner
Vette4cw