I've had my '93 6 speed coupe for a couple months now. I took it on a thousand mile interstate trip and have done some around town driving. I've noticed drivability problems. I did a search here and found a lot of hits about this - not surprising given the age of our cars (C4) and the tendency for electrical/electronic components to be more prone to fail as they age.

I figured I'd start a thread about mine. I'm NOT going to do anything about it right now. At this point I can still enjoy driving it, the problem isn't too pronounced, just a tendency to surge and/or sag at steady throttle settings and, at times, under heavy throttle.

In an effort to provide information others might find useful I'll post what I know now.

The PO was dealing with the problem. I assume his approach was throwing parts at it. In the glove box of the car was a little white lined tablet with the following information on it.

August 2015:
New Opti-spark distributor
New Opti-spark cap
New Plug wires
New spark plugs
8 new fuel injectors
2 new oxygen sensors
New coolant temp sensor
New coil
New ignition control module
New battery

I guess one good thing is those parts are probably good - at least I can hope so.

The PO had a lot of toys so he only put a few hundred miles on the car after all these parts were installed - in 2015. I haven't done anything yet because I wanted to put some miles on the car (run some gas through it) to get a feel for it, have some fun with it, and see what symptoms might pop up. Come winter I'm going to try to diagnose the problem. I'll get the cable and software I saw recommended for some data logging. There are just too many sensors and connections to try to out guess a problem. Since one of the reasons I wanted an old car was to play with it I guess I'm going to get my wish.

 

If your asking for help you need to explain what your problems are first. And data logging is a great way diagnose a problem.

The problem with someone else replacing parts is that many times they are a mismatch for what the engine controls need. Fuel injectors are a good example as your computer/ECM is programmed for injectors with a specific flow rate at a specific pressure. Changing injectors with different flow and pressure need an entire different tune stored in the ECM or fueling will not match the engine's need.

There are some very helpful books on OBD I troubleshooting along with a GM corvette service manual. I think this book by Ben Watson was very helpful (But I can't seem to locate it now at home):

How to Tune and Modify Chevrolet Fuel Injection

by Ben Watson


Hope this can help and good luck.

 

Thanks for your post Cardo0.

Thanks for the reference to that book, I'll look for it.

Right now I'm collecting seat of the pants data and putting it into the "meat computer", getting to know how my car behaves. I believe I have a fuel injection condition and I'm going to have to climb the learning cliff of how this system works. I'm going to update this thread with questions and information as it becomes relevant. Based on the results of a search I did today there appear to be more than a couple folks who have or are experiencing similar problems.

Edited to add:

I just found and purchased a copy of that book.

 

Good for you as I think that is one of the best on OBD I as the author details testing and troubleshooting each Chevy ignition and fuel control component individually with simple meters. I located my copy and can see I need to reorder as I wore mine quite a bit with at least 2 reads. Man that can be dry reading though and it took a long time to read it all twice.
Downside is book doesn't have much on the Opti Spark dizzy. But IMHO there is only 2 codes for that - one for high resolution and one for low resolution either one points to optispark failure.

Have fun.

 

When I started this thread I said I'd update it as time went on and I discovered things I thought worth mentioning.

I ordered a bluetooth dongle from 1320electronics and downloaded the ALDLAndroid app onto my phone. I got the dongle a couple days ago and today had a chance to attach it. I found the ALDL connector conveniently located right above the gas pedal . . . and still had trouble getting to it -- BECAUSE I'M OLD. Once I plugged it in (I ain't unplugging it till I'm good and done with this EFI problem) to the connector I plugged it into my cigarette lighter and Shazam!!!, it worked.

Once I reread the directions for the app I fired the car up and got the software connected to the dongle and logged for a few minutes. I got results and was quite pleased. Just before dark I decided to take it out for a ride and get a longer logging session which I did. I got the files onto my laptop. I opened the 'analog' file in Excel and saved it. I've attached the file here for your perusal. I don't know the significance of a lot of the data but I'm working my way through Ben Watson's book so maybe I'll figure it out.

If anyone want's to throw out some comments about the data feel free, I can use all the help I can get climbing this learning cliff.

During my drive to log this data I noticed that I got a new light on the DIC, "Service ASR" so I've got another issue to look into.

 

browsed through your XLS file, while i'm not any kind of expert it seems the timing is all over the place. I'm hoping to log my '92 6 speed also and was thinking of using the Actron 9690 to do it. maybe you could get a couple of separate logs, one just of it warming up and another of it warm and running it up through the gears

Bob

 

When I did my ride the car had been sitting for several hours since I did my initial idle test log. The coolant temp had dropped to 130 which was close to what it was when I did the test log which was the first start of the day when the coolant temp was 123.

You'll note 2 lines highlighted in yellow, 1292 - 1328. 1292 marks the start of an acceleration, starting in third gear at 37 mph, lasting 6 seconds and ending in 6th gear at 64 mph.

I noticed the EGR and carbon cannister were both showing activity which was a concern for me. As I mentioned in my previous post I don't know what to make of much of this data at this point but I do believe that the answer to my drivability issue, at least in part, will be found in logs like this. I suspect there are members here that can shed some light on this data, if they see it and if they have any inclination to provide input.

I printed out some pages from book 2, section 6E3-A of the manual that cover the engine control/emissions devices. It provides the parameters and explanations for the various devices, ECM controlled and informational. I've got a lot of learning to do.

 

I was just studying Book 2 of the manuals and noted the following quote in section 6E3-A, page 19:

"With the Tech 1 connected, the ASR system may be disabled and the 'service ASR' light may turn "on"

I guess the problem I mentioned in post #5 may not be a problem. It's amazing what you can discover by reading the manual. Hah!!! Some day I may even have some idea about what I'm doing with this car.

 

did you see lines 1293 through 1327 where the knock sensor was telling it to pull timing???

 

I've been taking more logs with the ALDLDroid app, just collecting data. I found the attached Tech 1 sheet in my manual and thought I'd run a log according to its directions and see what I got. I've also attached that log. I'm still not sure what I should be seeing between the integrators and the BLMs but I'm suspicious of the results there. Also given that the Tech 1 sheet specifies a range for the Injector Pulse Width of between 1 and 4 milliseconds at idle what I'm seeing at over 7 is way out and could certainly account for the richness.

A few minutes ago I spent a few minutes looking over the engine (my back is out so I can't spend much time bent over). I'm amazed that everything looks as good as it does on a 1993 engine with 110K miles on it. One obvious thing I did find was the rocker cover vent was out of its fitting at the rocker end so if it was sucking it was getting atmospheric pressure. That hose terminates at the throttle body.

We're getting thunder storms here so I won't be driving it today to see if it makes any difference and I'm thinking I might saddle up my old 1981 H-D FLT for a ride across PA route 6 so it might be a while before I find out if that simple, little vent tube being out was, at least in part, causing my drivability issue.

More to come.

 

Update to my post #10.

The rain held off today so I finally decided to take the 'vette out for a quick test ride to see if reinserting the rocker cover vent had any affect on the drivability issue. Of course as soon as I got on the main road it started raining - figures. Anyway, it does seem to have made a difference but I want to drive it some more before I draw any firm conclusion. I wanted to run another test log per the Tech 1 test process but when I tried to connect to the bluetooth device there was no power. I tested the lighter plug for the dongle and it worked so maybe I have a dead cigarette lighter socket in my car.

More to come later.

 

If you ever want to bring it down to Monmouth County I'd be more than happy to look at it for you...I usually do diagnostics on C5's and C6's and other cars but not on C4's but all you need is a basic understanding of diagnostics...the year doesn't matter !!

 

I just saw your post. Thank you for your kind offer. Hopefully I'll be able to work my way through the issue. One of my reasons for buying an old car was to keep my mind active. Nothing like trying to solve a difficult problem to keep the old neurons firing.

I'm also fortunate that I have a next door neighbor that worked in a GM dealership as a mechanic in the early 90s. One of the first things he did for me was use his Snap On (I think) MT2500 to try to help with my A/C problem. He's been a sounding board for discussing the problem. I also have another neighbor who's son is beginning his career as a mechanic who is developing a lot of experience working with engines. He's provided some useful suggestions one of which I'll post about below.

 

As I mentioned above the simple act or reinserting the right rocker cover breather tube made a significant difference in the drivability issue I've been experiencing. I want to continue to analyze my engine to be sure I have nothing else going on. I'm trying to get my neighbor, who worked as a mechanic in a GM dealership in the early 90s, to take my car out for a test drive to give his opinion about what I think I'm feeling.

My other neighbor, a young guy who's just starting off in his career as a mechanic, has a smoke generator, he lent me to pressurize my intake system with smoke to see if any other air leaks become apparent. I started working with it yesterday but I need to better seal the throttle body to prevent losing all my pressure. I only have a small, low powered pancake compressor and its volume isn't that great.

If I don't detect any other leaks I'll pull the EGR off the intake and have a look at it to see if it's carboned up and not sealing properly.

As an aside, the biggest problem I have working on my 'vette is it's so low. With my bad lower back I have to lean on one hand to do anything - sometimes you need 2 hands to work with. I'm going to have to figure out some way to get down to the car.

 

The EGR valve on the earlier L98 engines could be operated manually using a vacuum pump to pull the EGR valve open. It is the best first test to do to your EGR before tearing apart your engine!

Your data will take time to study and understand but I don't like to see the TPS go to 0 as many times as your does.

What is not clear to me is on line 1293 is you are showing 22* degrees where only seconds before the timing was up to 42* total timing. At 42* degrees the engine should be knocking not at the 22* when it started knocking. The CTS is rock solid so the car is not getting hotter and knocking. The MAP and Barometric pressures all stay the same so the load is stable.

Also why does the Left O2 and Right O2 come in that format? O2's send out between .1 and .9 Vdc when they are running. The signal oscillates back and forth from high to low and the reaction time in which it does it is important.

The newer OBD2 allows it to save which cylinder was knocking and individually reduce the cylinders timing by 2*.

I was not aware that our small blocks went to 42* timing, that seems high to me. I would guess from this data that something is not right with your Knock sensor. They do fail and according to a recent post they can even keep the car from running. My 1988 had a bad knock sensor and it was changed and suddenly the car had a bunch more seat of the pants acceleration.

I understand back pain as I have five crushed discs in my lower back. I have dealt with it since 1993. Trying to function normally when you hurt all the time can be frustrating. I am awaiting my left knee to work properly again which should be a couple days, I have a C3 with it's 4 speed Muncie attached to a 427 running 12.25-1 Compression ratio in an old 1968 C3 Convertible. Fun to drive, way too much car for these new drivers, no nannies to keep them from killing themselves.... Having a Clutch is like a Car Alarm.

Best regards,
Chris

 

Thanks for your reply Chris. I'm going out to play with my car soon. I want to try to get the smoke test set up and done and the engine put back together. I'll re-read your post later.

Re. the knock sensor, I've heard significant detonation a few times to the point that I backed off and that wasn't under hard acceleration so I've been looking at the timing in the logs as well. My problem is I don't have enough experience interpreting the data to draw what I would feel are valid conclusions. I've also got to troubleshoot my cigarette light socket, which seems to have given up the ghost, before I can collect any more logs. I'll have to read the manual to see if there are tests to perform to detect a bad knock sensor.

 

Okay Rusty, I had wanted to follow up on this thread but my knee surgery made me delay it. Don't feel like reading much on painkillers.

So looking at your data logs I don't see anything that jumps out as a problem. I see the ECM goes into Open Loop and Closed Loop as expected. Something you should note is the integrator/short term will vary/cycle when in Closed Loop so you can't use any one reading for the integrator/short term while in closed loop. But if the BLM/long term is other than 128/129 while in closed loop you can use that as an indication the ECM is adding or removing fuel to stay stoichiometric (14.7 A/F).

You will find many troubleshooting methods in Ben Watson's book but they all need symptoms to start with. The only symptoms for your drivability issue listed by you was "a tendency to surge and/or sag at steady throttle settings and, at times, under heavy throttle". Ben W. list's possible causes as follows:

-Transmission converter or A/C compressor clutch

-Vacuum leak

-restricted fuel filter or lines

-erratic fuel pressure

-Secondary ignition



Hope this can help.

 

Today I performed a pressurized smoke test of my intake system. I left it go for a while and built up a lot of smoke in the intake. I pulled the oil fill top out of the rocker cover and smoke billowed out so I know I had a goodly amount in the intake. The only place I saw any smoke escaping was around the ends of the throttle shaft. More smoke was coming out the throttle cable side than the other side. I also noted there was a lot of black combustion resudue in the bottom center chamber of the throttle body where, I believe, the IAC, valve enters the throttle body.

I'm going to study the manual to see if there are any seals on the ends of the throttle shaft and why I might be seeing unburned combustion products where the IAC flow enters the throttle body.

If anyone has input about these 2 things I'd love to hear it.

Thanks.

Randy

 

By the way, one of the previous owners of my car cut the louvers out of the air cleaner cover. I'm assuming the fact that my car has a MAP sensor instead of a MAF prevent that act from affecting drivability from the increased volume of air entering the intake, does anyone have an opinion about this? I have an IAT in the intake upstream of the throttle body but I'm assuming its reading of the air temperature isn't impacted by the removed louvers? Again anyone have a different take on this?

 

If the engine is having a hard time staying at or near the stoichiometric ratio then it sounds more like an O2 sensor that is lagging and causing the car to have to "search" for the perfect ratio. The knocking out of the blue on line 1283 was a surprise but that can be caused by a malfunctioning EGR as well as the incorrect fuel ratio caused by a slow O2. Try a Newer O2 in your car as it sounds like your car is in need of a working faster responding O2.

Your MAP sensor is likely to be a plus on your car. That is "only" if it works properly. The MAF sensor on my 1988 C4 is in the line between the air cleaner and the throttle body. If air can leak around you MAP or MAF sensor then your computer will have problems delivering smooth operation. It is critical that the air going into the engine is measured as it is one of the three important sensors for closed loop operation. I have had a MAP sensor fail in a Buick and there it was only under loads it would start to miss and buck a bit. After replacing it the car is back to normal.

Cutting louvers into the air filter lid will not hurt your MAF or MAP sensor. It was intended to get cooler air into the air filter and rarely makes much of a difference. I have the smooth tube instead of the corrugated pipe connecting the MAF to the throttle body (another .25 hp) but it seals well. Just remember you want a seal on the air going in so it does not leak in someplace else. Any air infiltration will mess up you A/F ratio via your MAP or MAF sensor.

The location of the IAT is frequently moved to try and read cooler air going in, another go-fast-er trick. I have mine in the factory location and works just fine.

Any more questions? Feel free as I am here pretty regularly. I apologize for my lack of experience on the newer engines that your Beautiful Corvette has! I am just going on my knowledge of fuel injection and C4's in general. Test the O2 if you have to but I am pretty sure it is the culprit in your car. Either that or something with your MAP sensor.

I wish you the very best in resolving the issues you are dealing with!

Best regards,
Chris

One more thing you might want to check is the timing of your engine. Have you set it yourself? I suspect it is a bit higher than normal which would explain the high total timing we saw in your data. On my car you have to disconnect a wire and then set the timing and then reconnect the wire to allow the computer to control it.