Been a long time since I have been out here, but I need some help with my 1993 LT1. A little background first.

The car is a 1993 Trans Am M6 with 62K original miles, engine never been modified or rebuilt. I originally bought this car in 1996 and have owned it since then with a short exception of two years when I sold it to my former BIL. The car used to get 26-27 mpg on the Hwy, even at 75 mph. Now, I seem to get 22 mpg at best. For years, the car seem to run fine, but mileage was never as good as it used to be. Last year, it started running terrible. I had a rattle in the exhaust for a while so I thought one of the baffles in the original muffler finally let go. This spring, I discovered that the catalytic converter had failed and the honeycomb brick had broken. I put a new Magnaflow direct fit cat and Y pipe on it and also replaced the entire exhaust from the cat back with a Flowmaster American Thunder system. Good thing I did as I found chunks of the converter in the exhaust and the muffer inlet at least 75% blocked.

Car seemed to run better at first, but now seems to almost have a miss. Fuel mileage is no better than it was with the plugged exhaust. I was sure that freeing up the exhaust was going to make this thing run like it used to.

I have a Tech 1, so I hooked it up and went for a ride. Everything looked good. The only question I have is the fuel trim. The left side (bank 1) has a LT number of 132. The right side has a LT number of 119. I realize these numbers are not far off the 128, but is it a bit odd that they are opposite? The left side seems to suggest a very slight lean condition while the right side is suggesting a slightly worse rich condition.

Given the poor fuel economy and the feeling like there is a miss, what am I looking for here?

 

What LTFT cell is that data from? There are 18 cells for idle, open loop driving, and various combinations of closed loop load (MAP) and engine RPM.

You mention short terms being different as well, but those should be changing very rapidly above and below 128. They should average 128 if the long terms are stabilized. 132 is +3%, within normal tolerance. But 119 means it’s pulling out (-)7%, and that¡s borderline. But it’s hard to determine the significance of the numbers without knowing what cell they are from, and what the numbers look like in other cells.

How old are the plugs and wires? Have the injectors ever been cleaned? Have the O2 sensors been replaced? Does the car sit unused for extended periods without use, perhaps compromising the fuel system? Have you checked fuel pressure? Do you know the ethanol content of the fuel you are using? The more ethanol the higher the fuel useage, and the more corrosion to the fuel system if engine is not run regularly.

For the past 8 years or so we've been recommending that people run a computer data log using Scan9495, a program written specifically for the OBD-1 LT1. Free download, you need a cable, the author is a member here and can help with install and setup issues.

https://www.camaroz28.com/forums/com...95-lt1-874306/

Unfortunately there are very few people who even come to this site any more. I'm pretty much the only one willing to review these data logs. Sometimes we find the problem, sometimes we don't. And the 93 ECM produces a bit less useful data than the 94/95 PCM. But it’s worth a try.

 

Plugs and wires were changed about 2005, but have low miles on them. AC Delco plugs and wires. I have run Seafoam through the fuel system by adding it to the tank and always add a can to the full tank of fuel when stored for the winter. I have been buying only ethanol free gas for a while now. The left bank O2 sensor was replaced when I did the Magnaflow cat, the right side is still original. Tech 1 showed both sensors very responsive and bouncing as they should. I did check fuel pressure with engine off and key on. Pressure was within range. I can do it again if you need an exact number.

The car does sit for a year or two sometimes. I have many cars, can't drive them all every year. It is stored in a heated building and always stored with a full tank of gas. Years ago, after the car sat for several years, it would not run on eight cylinders. I figured I had sticky injectors. Added a can of Seafoam and it seemed to clear it out.

Also, I pulled all eight injectors this spring thinking that I had put the wrong (later) injectors in the car when I had to replace some due to a small engine fire. All eight injectors are the correct ones for 1993. At that time, I found that the vacuum line to the fuel pressure regulator had broken. I had a vacuum leak and no vacuum to the regulator. I replaced the vacuum line at that time. I thought that might be my issue (this was before I did the exhaust). It appears that bank 2 is running rich and the computer is pulling fuel. Leaky injector(s)? Run another can of Seafoam or good quality injector cleaner?

I can get that program and a cable, but I do have a Tech 1 and also a Genysis scanner. Is there any other info that would help? Will the Tech 1 show me what cell the data is from? I did a snapshot on the Tech 1 to get the readings while I was driving at steady throttle in closed loop.

 

I can respond to some of this, mostly because I've left mine sitting for too long and have gone thru some of the issues noted here.

I'd avoid using fuel additives like Seafoam, Sta-Bil, B12 and the like. Those are bascially drying agents. Great for lawnmowers & pressure washers, but not so good for LT1s. Not sure where you get ethanol-free fuel, but these cars were designed to run fuels w/ ethanol--just not high ethanol (< 10%). They were meant to run high octane (92+)--another reason to avoid fuel additives.

Pressure @ KOEO isn't definitive, but it's a positive sign yours is within spec. What is the pressure @ warm idle? What do you get when you cap off the vac?

That yours sits for extended periods w/o being started (and put in gear) is an issue; been there, done that and am still kickin' myself in the azz. Have you replaced the fuel filter & FPR? The diaphram in the FPR can dry out and go bad, and the filter should be replaced when you leave it sit for extended periods. You can check for leaky injectors, but if it were me, I'd siphon out the old fuel and start w/ some clean 92+ octane. Actually, I did exactly that.

I'm not familiar w/ either of the diags you note, but even assuming they are serviceable, Scan9495 is the go-to around here; it's extensive on the data it collects (there is less on a 93, but only because of the ECM) and it's free. I got my cable for ~$12, but you can get 'em for less. Injun is the best I know @ reading the data, and wrote the book--literally--on reviewing all the various parameters. Dunno if he's familiar w/ the diags you note, but I know he can sail thru a Scan9495 data set. Just something to consider.

 

I doubt that a clogged filter is the problem, the two banks of injectors are showing opposite of each other with one side trending lean and the other rich. A plugged filter should cause the same condition for both sides. However, I agree that it should be changed, I will add it to the list of things to do. Changing the FPR is a good idea, it is still original, however, I again don't think that could be the problem either as both sides should be affected the same.

The Tech 1 is the GM scan tool that the dealers used to diagnose these vehicles back in the day. The Genysis is a more modern OTC scan tool that has much more capability than the Tech 1 has. The Genysis system will work on OBD1 and OBD II cars. It is good through 2018 or 2019 model year. It sounds like this other program is installed on a laptop and a cable is used that plugs into the ALDL.

I am thinking that my problem is a dirty injector or injectors on bank 2. I bought a bottle of Techron today and will see what happens after running it through. Your comment on the ethanol is interesting. I have used 92 octane E10 gas for years with no problems. However, when I run it in my LS1, I definitely get worse fuel economy. Several stations around here sell 91 octane that has no ethanol. When I run that in the LS1, I get 30 mpg every time on the highway. 93 is not required in these cars, a minimum of 91 octane is. I believe that most people have problems with their fuel when they store a car that has only partially full. The air causes the fuel to degrade. Having the tank full minimizes the the air that the fuel is exposed to. I have stored many of my cars for several years with full tanks and never had a fuel issue other than the one time with this car and I can't say for sure that it had a full tank when it was allowed to sit. I have had no issues using Sea Foam, but I never did like how my vehicles ran after storing them with Stabil in the tank.

 

I'd agree that the filter & FPR are not causing the specific issue you outlined initially, but it's still wise to swap 'em after letting it sit for so long. I let mine sit longer, and ended up needing to do signif. work on the fuel system. The fuel was still good, ended up putting it in my Jeep. There was ~1/4 tank in it, but it maintained pressure all that time, so I didn't need to swap the tank--thank gawd. Like you, mine is in a somewhat climate-controlled garage.

You might well have issues w/ the injectors, but pumping additives thru 'em is a poor sub for sending 'em out for a true cleaning (or replacing them altogether). I got lucky w/ mine, but I've never used anything in my fuel. I ended up just changing the o-rings & FPR. I use that stuff in my mower & pressure washer, but they both specifically state to empty the tank to bone-dry before storing (ironically, here in FL I have about 6 mos. where I don't need to cut grass--go fig). If the FSM doesn't call for it...

You are correct that Scan9495 runs on a laptop; the cable is a 12-pin to USB. You can actually make one easy enuff, but I got mine from someone local.

 

Check the FPR vacuum line for wet fuel. If the diaphragm leaks, and fuel enters the vacuum line it will be sucked mainly into the passenger side of the intake, because that’s where the vacuum connection is.

Agree, fuel filter will not cause split LTFT’s. Problems with idle atr can, although that is more common with aftermarket throttle bodies.

The fact that an O2 sensor is moving rapidly back and forth between rich and lean is not evidence that it is accurate. The values jump back and forth in response to the ECM toggling the A/F ratio back and forth between slightly rich and slightly lean, using the short term fuel trims. It does that so the catalytic converter will work. You could swap the sensors side-to-side to see if the problem follows the sensor. But you only have data from a single Cell of the fuel control system, and we don’t know which Cell it is.

Knowing the data from as many Cells as possible tells us a lot more. That's why the data log is much more useful than attempting to collect data off a scanner screen. The data in the PCM is changing close to 10 times per second. There are about 50 columns of data in a Scan9495 log (probably a few less on a 93). You have to look at many of the columns at once, which is easy with the log. Let’s say the problem is related to the EVAP system? We can look at the fuel trims and O2 sensors, and how they react when the EVAP system is activated, and even see if the problem is sensitive to the EVAP system duty cycle. Viewing the log on Excel allows the calculation of maximums, minimums, averages within the entire column of data, or over a shorter spam preceding, during, or after an event happens in another column.

In all honesty, as mentioned earlier, sometimes I find the problem, others I don’t.

 

What are you plugging into to run this program? The ALDL or directly to the ECM?

 

You need a cable that can run from a PC laptop USB to the 12-pin ALDL (aka DLC). This is Gary's past recommendation?

OBD Diagnostics

I asked him if that is still the best for anyone who doesn't want to build their own. And asked him if there are any updates to his D-I-Y parts sourcing.

 

Ok, thanks for the information. I put a bottle of Techron injector cleaner in the tank today. I have a road trip planned for Saturday that should go through most of the tank of gas. After that trip, I will check the LTFT again and see where it is at. If there is no improvement, I will get one of these cables and log some data.

I appreciate the help. I understand a lot of this, but am certainly no expert. I am better with Q-jets.

 

New plan, I won't be taking the car on a road trip tomorrow. I took it for a short ride today and once again, it smelled hot when I stopped. Not hot as in the engine is overheating (it's not) but hot as in that it smells like something is getting too warm. The logical part would be the exhaust since it is new. I know that new exhaust usually gives off an odor when new, but that goes away rather soon. I have a couple hundred miles on it now since the Flowmaster and cat were installed. Odor should have gone away by now. My concern is that I am running a bit rich and overheating the cat. Engine doesn't sound quite right either. Like I mentioned in the first post, it sounds like it has a miss. It also sounds like there is an exhaust leak under acceleration, but we have checked for that and can't seem to find one. There is no SES light or DTC.

Not sure what is going on here.

 

Here are some pics from the Genisys screen. First is at idle fully warmed up, next is at steady cruise closed loop, then idle again.

 

Let's talk about IDLE first.

It's in the idle cell (16) in first and third photos. At idle, there is a problem with one bank of the engine pulling out 15.6% (108 LTFT/BLM) of the fuel it would normally supply. But Bank 1 is not the RIGHT bank. Bank 1 is left bank, driver side, cyls 1/3/5/7.

Was the bottom photo take later, after driving for a while? The one bank has gone totally normal (128) and the other bank is still seriously screwed up (108).

Why did one of those numbers change? We might know more about what happened if we had a data log, showing thousands of lines of data, taken a fraction of a second apart. Then there's the possibility that the PCM has to pull even more fuel out, but we can't see the short term fuel trims (STFT = INT). A log would have those numbers side by side.

Then there is Cell 6,

That cell means the engine was operating in the range of 1,200-2,000 RPM, with a moderate engine load (MAP in the range of 30-50kPa)

In Cell 6 the situation hasn't changed much. One side is OK (slightly high) at 130 (adding 1.6% - within normal tolerance), other bank (we still don't know which, but your Tech 1 seems to have the banks correct)

Other:

PCM is in closed loop. MAP sensor appears to have set a rational barometric pressure in the PCM.

So.... why does the PCM have to subtract all this fuel from the amount it would normally supply? Could be a faulty O2 sensor (but we can't see the O2 sensor readings), Or leaking injector(s) on the mystery bank. Or... if "Bank 1 Right" really is the right bank, could be a leak in the fuel pressure regulator. Did you follow up wth my suggestion that you check for fuel in the fuel pressure regulator vacuum line, in post #7?

 

Yes, pulled the vacuum line off the regulator and using a hand vacuum pump, applied 10” of vacuum. Let it sit for a few minutes. No leak down, no fuel in the line. I also checked the impedance of all 8 injectors, all within spec at 12.7-13 ohms. Using the OTC Genisys, I can see the numbers are off in every fuel cell that I could observe. All are skewed the same, with one bank good and the other rich. I would say this is trending toward a leaking injector(s).

Yes, the bottom pic of the idle was taken after the steady cruise state for a few miles.

 

I should note as well that before I took it for a ride, I disconnected the battery and cleared the memory.