Sorry Mrvettenick, one more question then I will go away until tomorrow. So, low fuel pressure or fuel volume won't create a situation that some sensor might react to that will cause the ECM to send low injector pulse widths? Thanks

 

If the ECM had a block learn of say 150 or higher, that tells me the ECM is trying to add fuel. And that would be the case in a low fuel situation. Basically, the ECM would see the O2 sensor reading lean, and then it would compensate with a higher block learn/longer injector pulse width to increase fuel. It's not doing that based on your data. That is why I want to see if the ECM has the ability to adjust after it goes into closed loop. I'm not seeing that right now. You could create a small vacuum leak at idle jut to see if the block learn values/injector pulse width go up to compensate for that leak.

 

I would do a quick O2 response test....when your O2 is heated and in closed loop are you seeing the O2 go full rich (over 900 Mv) when spraying Brake Kleen or something similar into a vacuum line ??….is it stuck lean (I'm not sure if the 4 is 4 Mv's) at all times ??

 

The numbers I posted were from a few frames of a Scan Tool "Movie" I did when it was acting up. Prior to these frames the 02 sensor was reading all over the place from low to high as I drove it casually up the road. It leaned out (went to about 4) when I accelerated as if to pass someone and the torque converter unlocked. you can see that in frame 4 to 5. Can also see the Throttle Position Sensor voltage numbers advance. The engine just flattened out and was non responsive to a depressed throttle pedal. I can back out of the gas and it runs ok. It's just when I push the car to accelerate fast.
One time I was driving it when it was doing the above and I just kept my foot in it while it chugged and carried on. All of a sudden it caught hold and took off like a Corvette should. The next few times that day it was responsive. Next day same hesitation crap and dead throttle at say 80% or more throttle. It almost seems like a bad wire connection or short or open that is intermittent. I've physically checked all the wiring short of taking it all out of the car. I haven't read each wire out though. One thing (among several) that confound me is that I'm getting reading on all the sensors and can see what the ecm is putting out and the numbers make sense. I didn't work on it today and it may be a few days before I can get back to it. Mrvetteenick has some things for me to try too. The intermittent thing I mentioned above worries me that it is a bad connector or wire and the computer is getting bad information. Garbage in, Garbage out. Thank you for commenting.

 

The O2 sensor IS supposed to oscillate between 200-800 millivolts and like I had said at WOT the sensor should go full rich...over 900 mv’s. You should also see a 450mv bias voltage on your scan tool with the O2 unplugged...good video enclosed !!

 

Good post! My concern when you see data that looks "funny" make me question if it's the ECM telling the scan tool, or is it the scan tool's issue. When in a situation like that, I start doing some tests like the O2.....do you see .350-550 milivolts with it disconnected. Dos the coolant temp read -40 when it's disconnected, just to see what happens,
My big issue is the O2 will never get to 900 if the block learn in the ecm never goes above 128 at higher rpms.

 

I hate to be a problem but I see that people still "think" an Oxygen sensor can work "properly" and last over 30 years. They may still "work" but they are not working "their best" to help your Corvette get the maximum performance or mileage they can.

The signal strength goes down and the sensor lags behind in it's readings for the computer. The signal lag is the big problem as your computer no longer see's the instantaneously changing numbers and can adjust for them as quickly as is needed. Would you rather waste gasoline and loose power instead of spending $25 on a new Oxygen sensor? I don't understand the thinking as we drive our Corvette because of their performance, why use an old sensor that is no longer working as well as it did when new?

Don't try and clean a Oxygen sensor with any off the shelf chemicals. I have heard that people try testing them when the are out of the car and that is not a good thing either. There are proper methods listed online but they use propane to get a reaction from the O2 sensor. I don't even remove the plastic cap (over the sensor) until the unit is being installed.

I am not making this up. This is what was taught to us when we first started playing with EFI Corvettes. Just like the old 3000 mile oil change interval which I still use. I may waste motor oil but my engine is happy.

Having experienced an O2 failure in an OBD1 vehicle I am inclined to replace them before they die. I would not want any of my cars loosing the O2 while on the road as it could lead to an accident while your engine switches to Limp-Home mode. When my 1st O2 went bad the car intermittently would just stop making power and run like poop until the limp home feature kicks in. Not a good thing to happen in heavy traffic that we have here in Northern Virginia.

The quality of the Oxygen sensors has become better over the years but I still follow the recommendations I was taught. Being one of the MOST critical components in your Fuel system I will keep replacing them to ensure my Corvette runs it's best and I never have any more fail on the road.

P.S. The Oxygen sensor is supposed to oscillate back and forth from .1-.9 volts very, very quickly. If your O2 is not hitting the .9 then it might be time to try a new O2....

 

The best way to evaluate an O2 is cross counts on your scanner. Iv'e seen O2 sensors go a long way, and others not so long. I've also seen sensors that looked good, and didn't work. where ones that looked like crap performed like a champ. I agree that you shouldn't clean them. I've also seen people do a preventative maintenance on them, and the replacement failed early.
My only recommendation is if you don't replace it, renew the anti seize on the treads, so when it comes time to replace it, it'll be a lot easier.

 

I'm still working on my problem. While I have learned a lot I still don't know what some of you do. I don't know enough to fix it yet. I worked through the FSM and determined the EGR valve was bad so I replaced it. (It wouldn't hold vacuum with a MityVac but for a few seconds). That didn't fix it. In closed loop driving up the road it "flattens out" and won't take much throttle much less an aggressive throttle. I put a Bosch O2 sensor in it a couple of weeks ago. I don't understand the low (4) reading. The engine is lean but I didn't know the sensor would go down to 4 or was supposed to go that low. I did check the O2 reference voltage with my scan tool and Fluke meter. .395 volts. Pretty close to .450 and That's within range per the service manual for a 10 MegOhm digital meter. Below are some scan tool "movie" readings before I replaced the EGR valve. I may get some more tomorrow. I wish I could display a larger range of readings but it is pretty tedious taking them off the scan tool. What you see below is a very short period of time probably 10 or 15 seconds from the time I pressed the record button. I noticed it recorded a KNOCK at one scene then retarded the spark. Still the engine went very lean at a time when it should be going rich. Thanks

 

This is after I replaced the EGR valve (which I think did no good at all). This is a lot more information than I've posted before. I hope someone gets something from this. Frankly, I evidently don't understand the cars system well enough to know why it doesn't get more fuel when it should be. I say that because it appears to me it needs longer injector times but isn't getting it. Thank you for any input you might have.

 

Is your Knock sensor working properly? Believe it or not a Bad knock sensor can make a mess of a properly running engine. The knock sensor came on in line #4 above in your chart and from what I remember the system did just what it was designed to do. It pulls the timing back after hearing a knock then slowly re-applies the timing which is what you see on the chart. The engine does not look like it should have seen a Knock as the engine loading was not that serious.

I have a 1988 C4 and one of my neighbors has a 1987 C4 so we played around once and he could spin his tires easily with the automatic 700R4 which I have as well. My C4 would not spin a tire even when wet... It turned out that the Knock sensor was defective. Since the C4's computer "knew" it had a bad knock sensor it retarded the ignition to "protect" the engine. Now my C4 can spin the tires like other C4's. You can test the knock sensor by watching the output while you tap a metal wrench or other object on the engine block or cylinder head. It should react right away with a voltage jump.

When you replaced the EGR was it working completely normally or had it failed? Mine failed and was caught at a Emissions test so I replaced my EGR valve and the EGR solenoid mounted on the thermostat housing. The vacuum signal going through the solenoid is what makes the EGR valve move as needed. Be sure that the entire system is working properly after fixing it. A bad EGR will make the engine run hotter than normal and make it start to ping. When somebody mentions pinging my first guess is that they have a faulty EGR valve.

There are lots of people that simply disconnect the EGR and consider it "removed" from the system. The only way to remove the EGR is to get a new Prom with it removed in the software and remove all the components. The car runs so much better with a working EGR I kept mine functional. When I replaced the EGR on my 1988 it was a bigger job than expected due to the amount of carbon in the passageways. I had to de-carbnonize the parts from the engine that interacted with the EGR. My EGR failure started when the EGR Vacuum Solenoid failed and I could not notice the difference. The power wires to the Vacuum solenoid use PWM for controlling the EGR. I ended up replacing my injectors, FPR diaphragm, Vacuum hoses, Fuel Pump, Cold Start Injector and lots of gaskets. Today the engine runs much better and the mileage has been higher than ever. With my 700R4 going 60 mph on a highway the mileage goes above 30 mpg with cruise on and the A/C running. With everything working the Corvette is a pleasure to drive...

Look to see if you have an overactive knock sensor, that would explain a lot of that data in your chart!

 

Thank you for your thoughts. I will check the Knock sensor out. I do have an 88 service manual and I think there is a trouble shooting procedure to follow. The knock spark retard did decay but he engine never did deliver any more performance. Thanks for looking at my data and I will work on it.
Re the EGR: The service manual has suggestions for poor performance, sluggish, etc. about the only thing I hadn't done was completely check out the EGR system. I followed the manual and check the solenoid, the switch and the EGR valve. It was supposed to hold a vacuum for at least 20 seconds and it didn't come close. Maybe 10 seconds. I bought and installed an OE ACDelco valve and put it in. It didn't fix the issue but probably needed replacing anyway.
I'll check the Knock sensor. To me it's important that he injectors are not commanded to stay open longer when the pedal is on the floor. I read the computer reacts differently when it see a rapid and significant throttle. (stomping on it). It's supposed to tell the injectors to stay open longer but that isn't happening. I don't know if that is the cause of the low power or the result of something else telling the computer not to go to that block or cell or what ever it reads. It's about to get me down. Why would engines have to be this difficult to understand and repair?

 

With a knock sensor on the "fritz" the engine can do "strange" things. Your knock sensor went off at 2000 rpm and that is a little "weird" to me. I would bet your knock sensor is overactive OR there is something else banging hard enough for the computer to mistakenly "assume" it is detonation. Anything on the engine can trigger the knock sensor if it makes a metallic banging. When it failed my knock sensor kept my engine from hurting itself and made driving "less than exciting". It was not hard to swap out but do it when you are ready to flush your coolant. The knock sensor is one of the lowest point in the cooling system and will leak coolant when removed. When you remove the knock sensor you might not have any coolant leak if it is plugged with rust flakes, it has been known to happen...

IF your Corvette did what mine did where the fuel lines virtually corroded shut internally. Then I had enough Pressure but was not delivering sufficient fuel. If you have full pressure of say 42 psi and full fuel volume available then the time drops that the injectors are open. My 1988 C4 stays at 42 psi and runs great so there must be some issue with your fuel system. Be sure to change your fuel filter.

The EGR can be tested by using a vacuum pump and while applying a vacuum the valve stem will move up and down just a little bit. If you do it while the engine is running the idle should change. The whole system is controlled by the EGR Vacuum solenoid mounted on the thermostat housing bolts. The EGR Solenoid controls the EGR if it has sufficient vacuum to do so. I have sen a lot of cars with vacuum leaks that lead to EGR failures. The ECM controls the EGR with PWM and that can be measured best with a oscilloscope or other higher end measuring device. It is a fast pulsing signal basically. Be absolutely sure the EGR is functioning normally to have the best performance. Replace the vacuum hoses if they are original.

MAF issues are fairly common but I don't have enough experience to know whether your numbers are correct coming from the MAF on your Corvette. The ability to keep up measuring the volume of air being ingested is a critical job in Fuel Injection. When MAF's act up they send erroneous data which can cause drive-ability issues and shuddering and loss of power at speeds. Can someone out there in the Forum tell us if the MAF data he is getting is in the ballpark?

I know I am going out on a limb here suggesting this as there are many nay-Sayers who believe it is a snake oil... Try running a can of B&G 44K Fuel Injection System Cleaner through your system. I have found that the 44K works wonders on cars with injection issues. My DD started missing regularly and I put a bottle of 44K in and the problem was gone within 50 miles just last week. B&G 44K is awesome and it really works, I use nothing else in its place. In My humble opinion of course...

 

Hey !!
Rule of thumb for any car is basically the MAF numbers in grams/second is close to the engine displacement !!...my 5.7 roughly runs around 6 grams/second !!

 

I wanted to report back on the low power issue with my 88 Corvette. It was a bad "Mass Burn Off Control Relay". I didn't find it. A young man in town that has a trouble shooting business found it. I knew the MAF relays existed but never suspected them as I got what I thought were reasonable MAF reading on my Scan Tool. This is where experience and the right equipment comes in. He found a low voltage that didn't make sense with his oscilloscope. The contact points on the relay were bad. There are two MAF relays. One is the MAF Burn off Relay and the other is a MAF Burn Off Control Relay. The control relay was bad. I have learned a lot about this engine on my journey to the solution. I'm glad for that but not for the parts my son had to buy while I was working on the car. The symptoms all resembled things that can happen from several components. It says in the GM Service Manual a thorough understanding of the engine and systems operation are needed to accurately trouble shoot engine problems. I don't know if I would have ever gone to these relays on my own. Sometimes you just have to call in the pros. Thanks to everyone that offered suggestions and advice.