antfarmer2

Only played with one and it was tight.

JD1964

I can't figure out how to get the IAC count. The scanner I have isn't displaying it. Maybe it's not a feature of my particular scanner.

Check this link where I post more scan data. The data at this link was recorded after the TB was cleaned. Do you still see the same problem?

The car runs very well. ***** and gits quite well.

https://www.corvetteforum.com/forums/c4-scan-and-tune/3656854-96-lt1-scan-data.html#post1589695805

pkincy

Correct. Never to correct split BLMs and not on the LT TBs. I was mixing up my LS TBs that start with a small hole to help get air in and it needs to get bigger with a cam.

Your Bank 1 LTFTs are showing the pcm is pulling fuel. So check for a leaking or stuck open injector among other things such as an exhaust leak in Bank 1 before the O2 sensor. Something is making the front Bank 1 O2 sensor think the mixture is too rich.

BTW, my 383 LT1s with mild (CC 305) cams idle at 950 rpm, which I find perfectly OK. Any less than that and I run into surging issues.

Easiest quickest way I know of to check the injectors is to switch them from Bank 1 to Bank 2 and see if the rich condition moves.

I built a fuel injector tester some years ago with this (ACTRON CP7819 FUEL INJECTOR & HARNESS TESTER) and a couple of valves, some plastic tubing and a graduated cylinder, but that is likely more than you have in mind. There are also several plans around the internet for home building the same thing.

JD1964

Took it for a long drive yesterday. It ran great overall. However, it still delays to go back to idle sometimes. When it finally relaxes the idle is good at about 870-880.

If I'm cruising along in high gear at 1500rpm, I can take my foot off the gas but the car keeps moving without deceleration as if my foot is still on the gas. Even if I bump it to neutral while rolling, it will maintain the 1500rpm for at least 5 seconds before it begins to settle down towards idle.

If the EGR valve is sticking open longer than it should, could that cause this? Can I verify the EGR closed, then plug the vacuum line to it for a test? Will plugging the EGR vac line throw a DTC?

pkincy

It sounds like most of your mechanical issues are solved. Congratulations. Have you verified by logs that you don't have split LTFTs (the rest of us C4 types call them BLMs) any longer?

The Throttle Follower is a pcm thing. So I would take this last data back to PCM for Less and I would suspect they can fix it easily.

JD1964

I also adjusted the valves again. I originally had them 1/4 turn past zero lash but they were a bit noisey. Another 1/4 turn and they're much quieter now.

I'll let the PCM settle in for a period of time and then have a look at the data again.

So, are LTFT's associated with OBDII, whereas, BLM's are associated with OBD I?

pkincy

Exact same measurement simply different terminology. OBD1 called them Block Learn Multiplier and Short Term Integrator. OBDII calls them Short Term Fuel Trim and Long Term Fuel Trim (STFT/LTFT).

In a nutshell, the Block Learn Multiplier (BLM) is the long term fuel adjustment that the ECM "learns" to keep the air/fuel ratio within acceptable parameters. The Integrator is the same idea, only for short term adjustments. Basically, these two strategies are used to make adjustments and adaptations to the ever changing loads, atmospheric conditions, and fuel quality to keep the car's air/fuel ratio correct for driveability and emissions.
Also, keep in mind that when we say Long Term or Short Term, we are talking in computer time, not human time. In our ECM the Long Term fuel adjustment (BLM) happens about twice per second, while the Short Term fuel adjustment (INT) happens about 20 times per second (last quote from the Buick GN Website)

STFT/LTFT are expressed in percentages. Negative means that the computer is pulling fuel. BLM and Integrator are expressed in numbers from 108 to 160. 108 being rich and 160 being lean. Just a terminology change to confuse us. Even more so, my 94 had an OBDII connector but was an OBD1 ecm. You have OBD II so get to handle things a bit more like the LS crowd but have the mechanicals for the C4.

I will pull some of my 98 LS1 (now LS2 402) logs and see how bad your -13.6% Bank 1 LTFTs are based on my experience. I will do that later and get back to you. If anyone here has any experience that is more recent than mine please chime in re: the negative 13% LTFT on Bank 1 only.

pkincy

OK, I have reviewed some old logging runs on OBDII cars with heads, cam, headers some with stroker motors. Generally the LTFTs were with a percent or two on each bank. The pcm will control from +20% to -20% so your -13.6% reading is within the control range for that bank, but you still should not have one bank at 0% and the other at -13.6%.

I have mostly seen high negative LTFTs on deceleration. On WOT of course they go to Zero. But at idle you should have both banks within a percent or two of each other and somewhere in the -3 to+3% range so that the computer is in the middle of its control range in closed loop. At cruise you could get to a +6to8 and a -6to8% range and still be right in the sweet spot of the computer being able to do its job.

Keep chasing a leaky injector on Bank 1 or slight air leak in the exhaust precat on that same bank.

Sounds like you are pretty close to getting it purring. Having a logging tool for OBDII would be a good thing as your car gets older and needs more maintenance. That is generally laptop computer based with an ALDL OBDII connector and software in the computer.

If I recall the 96 Y body is a EE file, so I think you can use some of the free loggers as well as Datamaster. I don't even know if LT1Edit is still available with an EE def file, but check it out. I always found Datamaster easier to use altough I have both.

JD1964

pkincy,

Your posts are helping me understand things better.

Here's a 1 minute video of the fuel trims. It starts at idle then steps up in increments to about 3200 rpm then back to idle.

You'll see in the vid by the time it reaches the higher rpms, the trim is equal on both banks, whereas, at lower rpms and idle bank 1 is pulling fuel.

Does this confirm a leaking injector? Would a leaking injector be moot at higher fuel demand situations and that's why the PCM is balancing the trim at higher rpms?

For the life of me, I cant find any exhaust leaks prior to the O2 sensor. After all, exhaust leaks are pretty easy to find. I cant hear a leak condition whatsoever.

JD1964

I put a fuel pressure gauge on the rail and cycled the key on and off a few times. Turned the key off and noted 42psi. Over 10 minutes the pressure dropped 6psi. Is there anything indicative or abnormal about this?

pkincy

This certainly can be caused by a leaking injector. Other than spilling a little fuel when you pull up the fuel rail and injectors the switch of them from side to side is really pretty easy. You do need to get a little plastic tool to undo the fuel line but other than that it is a very short job. If one is leaking you will know because the rich condition will move with the injector change. There are a fairly short list of things that will typically cause one side to be richer than the other, but this is one that you can either confirm or not pretty quickly.

JD1964

Ive got the fuel line tool. How about this? I pull the rail, pull the injectors and stick them into the removed rail. Turn key on engine off and look for leaking injectors. Considering I have to do that to swap from side to side anyway, I figure if one or more are leaking, this will show which ones. Right?

antfarmer2

Leaking that fast you would have a bunch of fuel in your oil if not I would look at the rubber hose in your tank from the fuel pump or a bad fpr leaking into your vacumme.

Christi@n

Thanks

8a8mfh

Measure the tps voltage. Close the throttle blades almost completely shut. Slot the tps mounting holes and adjust it to read either the same voltage as before or around .60 to .67.

Ok now everyone else can quote this and tell me it's wrong.
It got my blms closer. That and raising the closed tps spark.

pkincy

And something like this is likely going to work. A negative 10% reading is well within the capability of the PCM to compensate. Also the LTFTs come together at more rpm, so we are talking about a very small leak. And it is not necessarily a leak, but as you say could be a sensor that is simply not spot on. But do google the causes of a (very slight) rich condition on one bank and you will get a lot of good ideas.

JD1964

If I do have a leaking injector, could that explain the delay in relaxing to idle after cruise? Such as, my LTFT's are more balanced at cruise since the fuel demand is higher and the leak is not as much of a factor. Then, I let off the gas but the leaking injector is still delivering fuel making the rpm hang higher. Shortly thereafter the O2 sensor tells the PCM the mixture is rich so the PCM pulls fuel from the bank until the O2 sensor is happy. Then, the idle settles to normal.

Does this assumption make sense?

pkincy

the thought process makes sense, but you won't know until you get it diagnosed.

there are some tuning things that you can do to get that decay to be faster. But first see if you can diagnose the mechanical problem and fix it. It is hard to tune around mechanical faults. Believe me I have tried.

I just rewatched your video and whatever is going on it is slight. The O2 sensor on Bank 1 seems plenty lively by looking at the STFTs, so that is not likely the problem. If it is a leak it is a small one.

Where you are going to have a problem is your next smog test. It is small enough that it likely won't be too much of a drivability problem.

JD1964

As for the smog test, as long as there's no DTC's on file, it should pass, correct?

There are no DTC's. The one that was there previously was because when I put things back together after the cam swap I had #4 and #6 ignition wires backwards at the opti cap. Once I figured that out, the light on the dash went out. I have since cleared that DTC from memory and non have returned.

I've got a refurbished set of Bosch III's 24lb from FIC on the way. Considering the cost of professional cleaning of injectors, I figured I'd skip the cleaning and just upgrade the injectors. I'll pull and post some more data when I get the new injectors in.

Thanks to everyone for contributing to this thread.

JD1964

The Bosch III's are in. That was an easy job. The idle characteristics have improved and so have the fuel trims (at least as far as I can assume). I put three vids of live scans up. 1) no load at various rpm. 2) moderate acceleration in 3rd gear. 3) gentle cruise in OD.

Tell me what you think compared to the previous scan vid.


No load various RPM


Moderate accel 3rd gear


Gentle cruise OD