Many years ago, I modded up a LT1 induction with 52mm tbody and used an L98 fuel regulator, remote stat housing, HEI up top in back with rear water plumbing routed similar to L98, L98 aluminum heads, and L98 can with roller rockers......and so it's developed some really crappy running issues recently.....

I have changed injectors, several times, and it's run with a 1227730 speed density computer with aftermarkets chips, I have 4 computers, and two different eprom/plug in chip sets.....

the issue is that it runs fine and grand many times, BUT most of the time it goes to running rich and idle/lo speed is **** poor rough, hear the exhause cracking sometimes it's so bad.....

It's heater 02 sensor, closed loop of course, reg/pump is set anywhere, from 30 psi to 40 psi, and when it's rough, the adjustment is no help.....

I"m going nutz for some months over this thing, checked wiring, timing, and of course the damn computer don't set any codes, but ti's OBD 1 of course, .....old code reader too, left over from the L98 daze.....

anyone any clues ??? getting desperate here.....see sig file for a few pix.....

thanks.....

 

Scanner or datalogger? Cat Converters?

 

Pull a few spark plugs. Are they carboned up? Check for a weak spark. Check for vacuum leaks to start. You have a MAP sensor?

 

NO, unfortunately.....no cats either, just headers into duals, with H pipe behind trans, free flow muffs....

Plugs looked normal, but changed anyway....I replaced cap/rotor button and even coil and module....no go, the HEI is run off the computer of course....timing is correct.....

another thread here told of computer programming chips blowing their minds with age....sounds like ME.....damnit......but I never heard of that symptom/happening.....

 

You need to invest in something to read data. Usually to get prom dialed in datalogs are sent to tuner. Where did proms come from.

 

I have 4 computers, all 1227730 speed density types, which I have rotated around on an irregular basis over they years, mounted by bolts/hanger behind the map pocket inside my '72.......thing that gripes me is apparently no codes being set.....now the programmer for one of the two modules/chips I have is Alvin Anderson....the other one by who, I dunno, I forget....Anderson has his name on his module set with his own ckt. board and it's spread out a bit, the other is just the black IC chip and another white looking ckt of near same size mounted with it end/end on the module insert......

Maybe I need get hold of Alvin on this....??

 

Proms are application specific. Tuner proms have been tweaked possibly preventing codes from setting. They are also largely application specific. Hypertech, Jet,....I'm sure there were others made generic l89 proms. They had minor changes to OEM programming because they still jad to meet EPA and CARB. If closed loop is turned off mostly likely codes won't set. You need local qualified tech or tuner to help you.

 

My silly scanner just says rich/lea on the O2 sensor, and it switches back/forth fairly constantly/rapidly.....but I pulled it out and cleaned/examined it....blew it out and so to test again....I really don't think it's the problem.....but lets go to the last thing, a broken wire in the harness.....it would set a code as to which sensor is messed up, NO????

 

Just how old is that oxygen sensor of yours? Oxygen sensors are "ignored" as long as they work. I have heard of people getting 15 plus years out the original. The engine really responds to a new sensor with a good solid signal. Usually the performance and the economy get better after installing a new O2 sensor. If yours is heated, that is great but the biggest question is how long has it been on the car? If over 2 years replace it and you should see better performance. It is a vitally important signal to make your car run properly

On OBD1 Cars the timetable on Oxygen sensors was replace every 24 months or 50k miles. This does not mean they die at that point but their accuracy goes to heck because they lag and the signal can start to narrow. In normal operation the O2 oscillates at a high speed give fresh signals as the car needs them. After time the signal is not as strong nor is it able to keep up with the demands made on it.

OBD2 cars are supposed to replace the oxygen sensors every 4 years or 100k miles. Heated or unheated they are the same.

Oxygen sensors are very susceptible to any contaminants such as oil, lead or other burning contaminants along with numerous other materials even the wrong brand RTV can poison the O2 . I keep the plastic cover on until I install the sensor just to be sure nothing gets on it. Cheap insurance..

 

isn’t that how the o2 sensor is supposed to operate?

 

YES, far as I know.....but the rate of switching back and forth I dunno nuttin'......and so that is the reason I never suspected it.....but with rich running, most of the time, what bothers me is just why in hell it would smooth out just fine for some time, and then go back to crappy......and NO CODES!!!!

I wood THINK a code wood be set if a broken wire...no??

at one point I thought I had it fixed with the MAP sensor on top of the plenum, found a retracted pin on the plug, fixed that and car ran fine for a day or so, but the issue came back.......been all over TPS, and one thing to note is the IAT/incoming air temp sensor is just tied to some wires below the air cleaner....the car at one time had up/over air induction, but that install was abandoned when I found WATER in the induction bellows/pipe just before the throttle body......so I removed it....so now with the K&N oval filter directly on the t-body, no place to mount the IAT sensor, ......

 

I have heard of many Oxygen sensors failing and making the car run rich. It is a better default than running lean and damaging the engine. Knock sensors can also affect your timing to keep your engine from hurting itself. A bad knock sensor kept my C4 from making the power it needed. After getting the knock sensor working properly the car ran much better.

We had an issue with a defective O2 on a 1990 Pontiac and it never set a code or recorded anything. Finally I was able to capture the time when the O2 was acting up on a portable software running on a laptop while driving. After seeing the proof on paper they agreed to replace it.

They say tapping on the body of the MAF while the engine is running is a way to see if there is a problem with the MAF sensor. If it cuts out or misfires then you probably have an issue with your MAF. If not look elsewhere.

The fact that the O2 is bouncing back and forth between lean and Rich is normal. The rate at which it does it is also important. This is why I made a big deal out of the age. If it has two miles on it but has been on the car for 5 years it is still time to replace it. If you connected the O2 to a oscilloscope you would see the oscillating back and forth but you would start to see the signal strength. From what I read about them O2's start to lag first and then the readings get worse. Instead of oscillating back and forth from 0.1v to 0.9 volts they start oscillating from 0.2 or 0.3 to 0.8 and the signal continues to degrade. Being one of the primary devices in your air/fuel system means you need to have it working perfectly ALL the time. Even testing it with another KNOWN GOOD O2 would help you remove it from your list of possibilities.

Installing the IAT can be done in a lot of places. I would find a nice place in a rubber or plastic hose feeding the air into your MAF. Install a grommet in the hole and then place the IAT in it. Don't make a hole in between the MAF and the throttle body or it will screw up the MAF calculations. I would mount it before the MAF in the air flow. On the C4's the IAT was installed in a bad place to start with. It shows more heat than actually should being installed in the intake plenum made of aluminum where it was put.

I also suggest to folks that when you replace the MAF sensor to replace both of the necessary relays to ensure everything is working properly. A bad relay will still cause issues for the MAF and make problems.

Clean electrical connections are also very important on the C4's. I bought a bottle of DeOxIt and it does a wonderful job of cleaning decades of corrosion away.

Verify that your Coolant Temperature Sensor is working properly as well. Intermittent issues with CTS's are rare, they generally fail reading too high or too low and lead to starting problems. The MAF, Oxygen sensor and CTS are the three critical sensors for a smooth running C4 L98. I use a Snap On MT 2500 to "watch" the activity inside my engine and control systems, it makes life really easy to see which sensor is acting up while running the Engine.

Best regards,
Chris

 

I want to say THANK YOU for the WONDERFUL HELP you guyz/gals? have given me so far, and I stopped by a hotrod shop here locally and he have me some advice also....

AND, although I never connected the two things, the fuel pump mounted to the rear frame was REALLY LOUD on the way home, and so I got to playing with it in the garage, and noted something about the pickup, and engine ran like pure crap, much worse than before..... I have this theory as to the noises it's been making and maybe it's sucking AIR into the fuel pickup and therefore compressed air along with the fuel into the engine.....because upon it raising HELL, I did a couple changes and it shut up immediately.....engine ran smooth too.....

I have more work to do tmrw on that bastard, I WILL WIN>>>>>>>>DAMNIT!!!!!!!

 

Well this AM, I spent over 2 hrs under the *** end of the '72, reminding myself of how I plumbed in the extra tank, and so noted 9 gallons still in the main tank, swapped around the hoses and car runs much better, ran it out on the main drag a mile or so, and back around the burb, and ran much better than before.....and pum is still a tad loud but not anything even CLOSE to what it was yesterday.....engine is liking life better, and the O2 sensor had been cleaned not replaced, it warms up fast and goes back and forth fairly quickly, like within 30 seconds or so....not so bad considering ~45f overnight lo.....

so the observation seems to be that up/over loop on the stock level sending unit and pickup in the car is going to be cut off down to tank bottom level, and float remounted to filler cap arrangement.....fix the bitch MY way,....
10 mpg really sux, but the way it's driven these daze, 5 miles is a long trip....in heavy traffic too boot, so maybe I can't LEGALLY bitch too loud, so I keep the bitch level down to a dull roar, no louder than the exhaust.....

 

I have never heard of cleaning an oxygen sensor. Just how old is the sensor? If it is older than 24 months it would get better mileage after replacing it. Even the heated sensors go bad, just because it heats up does not ensure it is working properly. Even bad O2's can go back and forth like they should, just not as fast as a new O2. The only negative here is that heated O2's cost a bit more than the un-heated variety. People use the heated ones so their car stays in closed loop at long stop lights, an older un-heated sensor would go into open loop at long lights at times. I have seen it on my C4.

My C3 with it's 427 gets better mileage than you are getting. and it is a Big Block with a Holley Double Pumper on it along with a 3.36 rear-end. That points to aggressive driving or a very poor air/fuel ratio which could be caused by a bad or OLD oxygen sensor.

Sucking air into a closed fuel loop is clearly not a good thing. You must be using an external fuel pump from the description. I have one on my 1968 C3 as well connected to an Aeromotive Electric Fuel Pump Controller that operates similar to the L98 fuel system. It is a Pulse Width Modulation type controller that upon power it runs for about 4 seconds to prime the system and then it shuts off and operates more slowly until it hits a pre-set engine speed where the fuel pump goes back to full time. It is great as it is "quiet".

I am getting ready to switch to EFI on my 427 so I will be using a new fuel tank that has the internal pump like the C4's and all other cars made today (it seems). I want a bullet proof fuel system so I run -8AN SS lined Braided fuel lines from the tank to the engine. I will be adding a new -6AN SS braided return line to properly use the newer EFI systems.

Why do you have multiple tanks for gasoline? I have seen race cars with a tank of street gas and a tank of race gas.

 

Gene, you really need a data logger to start with. Sorry but I think you have proved to yourself troubleshooting EFI by trial and error is not effective let alone troubleshooting an EFI swap. But good news is there's some free data logging software to start with. Just need to search for it - maybe a post on this forum notes it. FreeScan or WinALDL maybe?

Good luck.

 

Please don't forget guys, it's a '72 vette and a replacement tank by previous owner.....but stock setup, and so when doing the FI install, I noted when the tank was lo, take a corner and the slosh made engine stumble and even die once in a while....so my cure was a small tank under/forward of it.....

but I have come to the conclusion that the plumbing was a mess from the get go some years ago, but because my highway driving is limited, so for some reason it's been raising HELL recently....been running it a bit with just the main tank feed, pump is more 'normal' sounding now, but I still wonder.....engine seems happy....talk about an off the wall kind of crap experience.....I suspect it's due to my driving habits/experience through the burb here, I take corners quickly, so IF the fuel still accumulates air in the fuel feed line, well pump noise and engine very unhappy.....I would THINK the O2 sensor would pick up and tell computer something rong, but......nothing on my very olde tyme scanner/reader......so to run tank dry enough to change the pickup around and then see WTF goes on.....still fishing, in other words.....