Hello to everybody. I just purchased a 1985 Corvette 4+3 with 36,000 miles Red/Red Leather W/plexiglass see through top. The body is in great shape but the car is running really rich. I replaced the O2 sensor but no change. I am going to try to track it down tomorrow. My question is how does the MPG guage get it's reading? My guage always shows 0 and the average always shows 99. Every other guage works perfectly including the gas guage. I read some where that it is controlled by vacuum, if this is true than it might explain the running rich problem. Any help would be much appreciated. By the way it has new plugs, new o2 (there is only 1 right?). It runs fine, no stumbling, no skipping, just black smoke and the wonderful smell of gas. I will get some pics in my profile tomorrow. Love the car, just don't like the aftermarket spoiler (gonna look into taking it off once I get it running right).

 

Welcome to the Forum.

You might pull the small hose from the fuel pressure regulator and see if there is fuel in the hose. This would indicate a broken diaphram in the FPR which could lead to running too rich.

Do you know how to pull the codes? Are you getting any?

 

I haven't really gotten into it yet. I was reading up on possible causes through the search, I have a nice long list that will keep my busy tomorrow. Fuel pressure bleed down test, compression test (although I would think it would run like crap if any of the cylinders had bad compression), Injector voltage test, Ect, Ect. Hearing that vacuum leaks can cause a rich problem, and that the mpg guage could possibly be controlled through vacuum. I would love to know how the guage is operated and where the vacuum lines are located (if that is what controls it), it could save me a ton of work. Also the driver side exhaust manifold is leaking. Is the gasket/or manifold as easy to change as it looks?

 

I'm no help on what may be wrong....I will say welcome to the forums and NICE find by the way....

With 36k on the clock it has to be something simple..something has malfunctioned and once that is found and repaired it should be just fine

 

the mileage indicator is an electronic computation.
THe computer takes the throttle position voltage,the vehicle speed sensor voltage, the oxygen sensor voltage, the pulse rate for the injectors, the pulse time for the injectors, the mass air sensor feedback voltage, and those make a composite voltage. that voltage then equates to a look up table value that is converted to a voltage to give instant readout.
The average mileage is a composite equation of time vs distance, instant average speed, etc.
or in other words, it's my guess.

 

I think you'll have lots of fun with that find. Be sure to sell some furniture, rob you IRA, whatever but get the factory service manual for that car. You will be an expert in about 10hrs after reading the general chapter introductions. Really. Another thing to do is read the owners manual. I drove my car over a 1000 miles and didn't know I needed a special tool to change a flat tire. Lucky and dumb me.

Again, welcome to the foruma and enjoy your car.

 

Yes, getting the FSM is a good suggestion. Also there are some really good books on fuel injecton that would be helpful.

Start by simply checking the fuel pressure and pulling the codes. Do you know how to do both of these things? The first requires a fuel pressure gauge, but they're relatively inexpensive and readily available at the parts store. The second can be done with a paper clip for a jumper.

Is the SES light on? Does it come on when you turn the ignition switch on, with engine off? Since the car is in its current state, it's very likely that someone removed the SES light bulb to sell the car. You will need to replace the bulb so that you can draw the codes without buying expensive test equipment. The codes can be drawn by shorting pins A and B in the connector below the dash, turning the key to the on position and watching the SES light flash. SOooo.... without the light bulb working, you will be shooting in the dark.

I've never researched the details, but I've always assumed that the MPG calculation was a simple one made by input from the fuel flow sensor and the Vehicle Speed Sensor. From that it is also calculating a running average. This might very well be a bad fuel flow sensor or some such.

If you are the type person that gets into doing research and figuring things out, I think you have a fun and very rewarding project ahead of you. Keep us closely posted and we will help you through it.

Doc

 

Fuel pressure test is
38 Running
40 after shut off
36 after 5 min
32 after another 5 minutes

Is this normal or is this indicative of a leaky or bad injector? Also How do you identify an injector? I have a brand new set of 24/hr injectors from a L98 TPI camaro I had. The box is long gone and I can't remember where I got them from. They have 010(or D)1048(or B) stamped on the side and 3537 somewhere else.

 

One other thing. I tried the paper clip trick and got nothing. The check engine light is on but it will not flash. Check engine light not flashing, extremely rich, mpg gauge always reads 0 while every other gauge works fine. Bad ECM maybe???

 

You're jumping to extremes. I don't blame you as I usually think the worse....but.....

 

You know, in Electronics when a system is doing all sorts of weird things, it's time to check the power supply voltage. Voltage regulation is something I've never looked into on an FI system. I would expect that there is a voltage regulator on the ECM board itself.

For grins and giggles, take a DVM and just measure the voltage across the battery with the engine running. Along those lines is the Voltage readout on the dash working and does it match what you read with the DVM?

Also, your ground connections are power supply related. This is where a factory service manual would be quite helpful for you so that you could identify all of them so that you could check them.

Digital electronics typically operate on 5VDC. I expect that a regulator is on the ECM that regulates the supply voltage to 5VDC. It may very well also have 12 volts more precisely regulated for dealing with sensor inputs.

Do you have any electronics knowledge at all?

Doc

 

Okay, they of course do not publish ECM circuit board schematics, but I think I see the best way to check the ECM power supply voltage.

Use a DVM and check the grey wire at the TPS referenced to ground. The TPS is the Throttle Position Sensor at the passenger side of the Throttle Body. There is a three wire connector and you can get a test probe into the back of the connector. Put one test probe in the grey wire, I think that's the top wire of the TPS connector and the other test probe to ground and you should have VERY close to 5 Volts. If you don't, try the other two connectors to make sure, since my manual is for a different year car. All that's in there is a potentiometer, so one is a solid 5 Volt reference, the middle one the wiper that changes voltage with throttle position and the third one to ground.

The grey (I think top) wire is the 5 Volt reference that comes from the ECM. If it is very close to 5Volts at that point, then Voltage Regulation on the ECM should be correct. If it is below about 4.9 or above about 5.1, then the ECM board, or more specifically 5 Volt DC Regulation on the board, would be suspect.

My FSM is for an 88 and these cars changed surprisingly from year to year, but I think that the TPS reference voltage check will be valid for yours.

Also, to verify the SES light, you shorted A & B and then turned the key switch on, but did not start engine, correct? If that is correct, then something is really amiss. The light should flash once, pause then twice. Repeats it a time or two and then flash any codes that are stored followed by the once then twice routine. If it is not coming on at all when A & B shorted and key on, the ECM would again be suspect.

NOW, if you don't have 5 volts reference at the TPS, don't just run out and buy an ECM board. Check the input voltage and your connectors first. You also should see 5 to 6 volts across pins A & B on the ALDL when the key is turned on.

All these are preliminary tests. You really need the book in front of you to go through some specific tests that they have.

You should make sure that all your connectors are on the ECM correctly and fully seated.

Krynick, it really sounds like you are going to find something like a bad connection or bad card somewhere and make a jewel out of this low mileage car you bought. Don't give up!



Okay more thoughts. I am much more familiar with other manufacturers FI systems and all of them have what is called "Limp Mode." This allows the engine to run, albeit VERY poorly so that you get home. I am wondering if someone might have removed the PROM chip. That would be the same as disabling the ECM which would possibly put it in Limp Mode. That might be a crazy thought, but it wouldn't hurt to check the PROM chip and make sure that it doesn't have a bent pin.

Can you get to the ECM very easily after making these checks?

Doc

 

BTW, those fuel pressure readings don't sound bad and they certainly aren't so far off as to make it run so rich. I think your problem is electronic.

 

Doc, no such thing as a fuel flow sensor on an 85 Vette (or any Vette for that matter).

 

Okay then I fully expect that fuel flow is calculated by fuel injection pulse duration. The MPG gauge on these cars are only accurate within about 10% or so anyway. It's something I've never thought about before except for thinking that it has to be based on fuel flow and distance traveled.

 

Thanks for the great replies. I am no electronics genius but I can get bye. So you saying that the pressure I am showing rules out leaky or bad injectors. I am going to do a compression test tomorrow just for $%^^ and Grins, although I don't think that is the problem. MBDiagMan I am going to go over your replies a couple of times and see if I can get a good inderstanding of what I am doing. I have to go out and buy a multimeter tomorrow so we'll see how far I can get with that.


That is exactly what I did. After I got nothing I tried to reset the computer by unhooking the battery. The Light never shut off. I am going to pull the computer out now and look at it. Is there any way to see visually if there is a problem with it?

 

Generally you can't find circuit board problems without electrical measurments, but never overlook the value of a good visual inspection. There will be a large Integrated Circuit. That is a Read Only Memory chip and has parameters that the computer uses. It is often changed for one that has different parameters for added performance. Make sure it is installed well and that no pins are bent. People who are not accustomed to dealing with removing and replacing them will often bend a pin or more than one for that matter. Also, there will be a notch in one end of the chip, or a dot on one of the corner pins. That notch or dot is indicating pin 1. Make sure that it is in pin one of the socket. People can get them in backwards.

It will be worth examining and also making sure that everything is plugged in good. You never know, you might see something burnt.

When you get a multimeter, get a Digital one and make sure it has input impedance of about 7.5 MegOhms or more. You can get a pretty good little digital multimeter at Sears for about $20 these days. It's Chinese made and not the best in the world but will get the job done for you.

It will be plenty good enough for checking the Voltages we discussed, and will be good for checking some of your other connections and your alternator voltage.

Doc

 

Well I checked out the ECM and everything seems to visually be in order. I am going to get the multi meter tomorrow and see if I can figure some of this out.

 

Well TPS checked out @ 5.0

Battery 14.4
Gauge 14.6

I would think that should be close enough.

The A + B however alternated between 3.9 & 4.0 never any higher.

 

The only thing about those measurements that is out of whack is the pin B. If it weren't for the fact that you can't get it in diag mode, I wouldn't think much of it. Since you can't get it in diag mode AND the diag voltage itself is low, that appears to be a clue.

Where is your ECM, behind the passenger side dash? Is it easy to get to the connector on the ECM with a meter probe?

It sure would be nice to have a known good ECM laying around to plug in and try.

I wonder if anyone on the forum has access to an ECM schematic?

Doc