Masochist

iTrader: (1)

My 383 is brand new and entirely rebuilt with LE2 heads and a custom grind blower cam boosting 10#. Just finishing the break in cycle at 1500 miles. About to swap in synthetic oil and get it tuned.

What worries me right now is that the car idles fine when it is cold and running what I can only assume is "closed loop" mode. Car runs like a champ during this time actually. Let 5-10 minutes go by and she just straight starts running like **** in what I can only assume is "open loop" mode. Anything between 15% and 30% throttle the fuel is just not working right, it either shoots in spurts of fuel or leans out entirely under load. My A/F gauge is sitting there freaking out under load and usually when this time hits when I'm on the road I book *** back to the garage for fear of damaging a component, seeing as how everything in there is so raw still. Now a factor here is that it is VERY cold outside, the two times this has happened to me it has been 15 degrees or so. So that may play into it.

Now the car has NOT been tuned. It was given a PCMForLess tune, if you can call that a tune, that did nothing but make it run like absolute ****. I'm not sure what happened on their end as all the information on my end was dead accurate, but thats none of my concern as I will never be a customer of theirs again. This is aside from the point, however..

I am fairly certain that the tune from a local LTx tuner will fix it, but I am not absolutely certain. So that brings me to my question, will the tune fix that issue? Or should I be troubleshooting something else perhaps before I get the tune?

There is a possible exhaust leak as the car tends to smell like gas, even though it has a hi-flo Magnaflow cat on it... and the engine is not making much vacuum being a 4L60E and the brakes are weak at lower speeds.

Thanks for any advice anyone can give.

Tony Shepherd

iTrader: (4)

Sounds like me its in the tune.

You will be in open loop until you hit a certain temp and the O2 sensors heat up and can start monitoring. Then it will hit closed loop.

Proper tuning should fix it.

Masochist

iTrader: (1)

Ah, so its opposite of what I was thinking. It runs in "open loop" until it warms up and then hits "closed loop", correct?

Thank you for the assurance that the tune will solve this issue, I am close to getting this car running properly, I feel a lot better knowing that there isn't a budding issue.


Now if somebody could perhaps answer why my car smells like gas, could it also be because of the tune? I get out of it after 10 minutes and can literally smell the fumes on myself as I'm walking. Any ideas?

theroad64

iTrader: (1)

Think of the PCM/sensor system as an electrical circuit that isnt complete until the 02s warm up, and a couple of other conditions are met.

Then it "closes the loop" completing the circuit.

infinitebird

iTrader: (11)

Sorry, but I must disagree 100% lol.

Tuning will NOT I repeat will NOT solve your issue. It is a common misconception that you can tune closed loop A/F. The computer always attempts to achieve 14.7 in closed loop. This is not adjustable. The only modes you can tune A/F in are wot and open loop.

It sounds to me like your O2 sensors are bad. What you describe is exactly the symptoms of that.

There are other causes of running rich such as a bad MAF sensor, other issues that are related to tuning, but the fact that it runs fine in open loop points to the likelihood that it is not one of those.

Perhaps a better explanation will make this more clear. In a sense open loop is ONLY using the parameters in your tune, in addition to the MAF sensor reading. So if it runs fine in open loop, you know that your tune is basically correct. Closed loop is a mode that tries to ascertain how "correct" the tune is by bringing in additional data from the O2 sensors. Their readings lets the computer know how close to 14.7 the calculations it made were. If they were off (meaning the tune or MAF reading was not correct in some way) then it makes adjustments accordingly.

This is also an example of why its good to find out exactly what is wrong in a situation like this before coming out and blaming some one. pcmforless has done enough tunes that I highly doubt they are to blame. If they totally fuxored your tune to the point that it would run like that (maybe messing up the injector constant or something like that) then it would run bad all the time, not just in closed loop.

Masochist

iTrader: (1)

Interesting...

I can definitely believe that the 02's could have gone bad. Could this also be linked to the gasoline aroma issue? I will have to look into this ASAP, is there a way to tell if they are cooked? Will the sensor look obviously fuxed? Or will there be other signs to look for incase of a failure like this?


If it is how you say, then perhaps I have spoken too soon on PCM4Less, that would make me happy, actually. I had faith in them before this and was utterly let down when she fired up for the first time and began running like garbage from then on. I have been questioning my stance on them lately, if this is true then I will have to totally change sides on this subject.

Sir, if you have any more advice, please let it roll.

infinitebird

iTrader: (11)

If you have a datalogging program you can check. In a data readout, the sensors should fluctuate rapidly between 100 and 900. If it looks like they are just staying around one small range of values, or if they read a constant value or do anything else but rapidly fluctuate in that range, then they are most likely dead. One issue with obd1 cars is that they will often not detect this and set a code. obd2 cars are better at this.

Another thing you can try is unplugging one or both of the o2 sensors. This will keep the car in open loop all the time. If it runs well then (even after 5-10 min etc), that pretty much shows conclusively that the problem lies there.

Masochist

iTrader: (1)

My '95 managed to come factory with OBDII. I will go in and unplug the '02 from the bung and see how shes runs.

It shows a lot of the signs of an untuned H/C car, she tries to a die while stopping from time to time (only in closed loop) and has an uneven idle at 1100 in closed loop, with an occasionally bouncy 850-950rpm in open loop mode. If I unplug one or both and it starts to run well, I am going to have a heart attack of happiness, I think. I'll keep you posted.

infinitebird

iTrader: (11)

well even with OBDII it's not guaranteed or anything, but you will get error codes more often with o2 sensor problems. Also, does it run well at wot or not? I noticed you said "anything between 15% and 30%" throttle. Does it run better with more?

Masochist

iTrader: (1)

I will do a 100% WOT pull and tell you what happens, most I have gotten on it is a 75% throttle pull and it actually did just fine w/o backfiring or anything. I am not sure if "backfire" is the word exactly, it just pops oddly and the A/F gauge either leans out fully or goes 100% rich.

If I can give you any more info that will shed some light onto the situation, I will be glad to do so!

infinitebird

iTrader: (11)

Well I was just asking because if it runs fine at wot that is yet another indication that the o2 sensors are the problem, since they are not used at wot. no need to do 100%, 75% is already "wot mode" to the computer in that sense.

Basically the only time the o2 sensors are used are warm idle and cruise, which is exactly where you say it runs bad. open loop and wot are usually unaffected even if they go bad.

btw since you obviously need to replace them, order ac delcos not bosch lol
https://ls1tech.com/forums/lt1-lt4-modifications/842270-honest-opinions-bosch-o2s.html

Masochist

iTrader: (1)

Hahaha, I'll keep that in mind, thanks for the tip. When I get home from work I'm going to pull out the bung on one of those 02s and see what happens.

You've helped me significantly thus far, my friend, now can you perhaps shed some light as to why I smell like gas when I get out of my car?

infinitebird

iTrader: (11)

Probably because it is running pig rich lol

that's what happens when your o2 sensors crap out

BTW, since you have an obd2 (with rear sensors) car everything I've said in this thread only applies to the 2 front sensors, in case you were not aware of that.

Masochist

iTrader: (1)

Yeah, the rears are for the cats, are they not? This car still has cats as I said earlier, if the rears were defunct could that possibly cause a problem? To my knowledge the fronts are the ones that matter not so much the rear, but could they still 'cause a problem?

infinitebird

iTrader: (11)

the rears going bad will not cause any drivability issues, it will just set a code usually.

speed_demon24

iTrader: (6)

That couldn't be farther from the truth.

infinitebird

iTrader: (11)

lmao, so would you care to explain to me how it really works then? Say I want my car to run at 15.5 in closed loop. How would I tune for that?

I am not a professional tuner, just a hobbyist, but I know quite well how the process works. Otherwise I would not be tuning other people's cars.

There is absolutely no way to adjust the target a/f ratio in only closed loop through tuning (as you can with open loop and wot). The pcm is hard coded to always target 14.7 in closed loop. The o2 sensors are used to achieve this. From everything he has stated in this thread, its quite clear his problem is the o2 sensors and not anything related to tuning. So even if that were possible to change that, tuning would not fix the problem of a malfunctioning o2 sensor.

But since you believe this kind of adjustment to be possible, please tell me how you would accomplish it.

No offense but, your extremely short dismissive reply devoid of any useful information leads me to believe that you don't know what you are talking about and just share that common misconception that I mentioned.

skinnies

iTrader: (33)

I agree with it sounding like the o2's are bad, you need to scan it and see what it is doing. A tune will definitely help it, but when my right o2 started freaking out it would dump in so much fuel it'd drop my afr to 10.0 at idle and try to kill the car. Threw it back in open loop and it was fine and upon looking at the datalog it was correcting like CRAZY because of the bad o2 sensor.

You can screw up the part throttle tune so bad that it won't run right in closed loop, but i do believe this issue is most likely an o2 sensor problem.

infinitebird

iTrader: (11)

While this is technically correct, what I'm saying is that there are no part throttle adjustment parameters that only affect closed loop.

Proper part throttle tuning basically consists of setting the timing right and in the case of a street tune, making adjustments to the MAF sensor tables (or VE tables for speed density) to get the fuel trims as close to 14.7 as possible. However both of these things will affect how the car drives in all modes and not just closed loop (since timing and the MAF are used everywhere), and they offer no capacity to target an a/f ratio other than 14.7.

This is why if you are having fueling problems in only closed loop, the problem is your o2 sensors and not tuning. I'm just trying to dispel the myth that tuning will fix this kind of issue because I see that suggestion repeated a lot on here for this kind of problem, especially if the person has a modded car.

Tony Shepherd

iTrader: (4)

I am a tuner myself and have more experience than most. Yes you are correct bad O2's would cause false readings in closed loop therefore you would be beating a dead horse trying to drivability tune it that way. But I think there is more to it than that. If I read his first post correctly he was given a tune? Maybe it was not for his car or combo?

Try to record some datalogging on the car and email it to me. I would like to see the O2 mv readings and BLM'S.