madmatster

Hey gang,

i have a 91 6speed z07 with a fresh 383 bottom end. It's got a MR/aluminum TPIS heads, etc.

In august, i had a lifter tear the lobe up on my cam, so i had the motor torn out and gone through. My builder changed my cam (original had too much lift in his opinion). I've used PCM of NC to do my tuning. (mail order) Just been datalogging and sending them logs/back and forth they have been burning me chips.

The car runs great on the road, but during wot, it bucks a bit going through the rpm range. PCM of NC has looked at the tune closely and says it looks fine. They've been very helpful during the entire 2 month long process of getting the car to where it is today.

I don't know what to look at now. What could be causing this "bucking". I changed plugs/gaps (seems to buck less with a .50 gap)

If i wasn't in NJ, i'd bring the car to them and have them go through it. Noone in this neck of the woods has good experience with the C4 anymore.

What do the wizards here suggest i look at? anything obvious I can focus on?

thanks

slow-vette

My first thoughts would be to verify fuel pressure, then check the TPS for proper function, Then check base timing and make sure you aren't over advanced and having the knock sensor pull timing. If all of that checks out take a look at your coil. I just solved this issue on my Brother in laws 85. It idled great everything checked out just fine but at about 1/2 throttle and as the RPM's started going over about 2200 it would cough, sputter, and buck like craze. I put in a new coil and that solved all of his issues.

madmatster

Timing is good (6 degrees). I do like your thoughts on the TPS. it sounds like you can toast these sensors by getting them wet. (i did wash the engine bay after i got the car back and before all the tuning). I'm going to pick up a sensor and try that first.

next step i'll check fuel pressure (not sure how i can check while i'm at wot, unless i install a remote gauge).

i appreciate your input.. thanks..

VikingTrad3r

if you own a digital volt meter, save yourself time and money by testing the tps before spending the time buying and installing a new one. search the test process here its super easy.

slow-vette

I'll even save you the search time. Key on Engine off. Probe the top 2 wires and check for .54 volts at idle. Has to be over 4v at WOT or the computer won't go into fuel enrichment. If those are correct then slowly open the throttle and make sure that the voltage goes smooth. If it goes up then down then back up again it's bad.

madmatster

looking at my latest datalogs... during some wide open pulls (TPS 100%), i can see my TPS volts reading never gets above 3.9. From what i'm reading, it should be well over 4 (or 5) for WOT.

i see that during idle and low RPM i'm at around .5 volts. (where the car behaves very well)

I would think this means, there is a good possibility the sensor.

at least i think. i'm going grab a new one.. unless someone chimes in and tells me i'm an idiot.

VikingTrad3r

3.9 is pretty close to "over 4" so im not sure if the computer literally needs "over 4".

either way id test it.

Buccaneer

I'm not a owner of a 91 C4 or any C4 for that matter, but I do have a lot of tuning experience and tuning is tuning no matter what the year, just the amount of things to play with the newer the car.

Have you ruled out anything yet? If you are data logging, you should be able to see something that is wrong. I assume you are running a WB O2, right? If so, what is the A/F at WOT, does it drop low or go high at some point? You should also be able to see what the knock sensor is doing in relation to timing. If the timing is being pulled because of some type of picked up or perceived knock, you will see that. I wouldn't get hung up on the TPS unless when you are measuring the voltage while rotating it slowly you see it drop (dead spot) or spike badly. It should be a smooth voltage range.

Also, did this just happen out of nowhere, what was the last thing you did before this happened? What is the injector pulse doing? I hope this may help you a little, but it may not if you are not really familiar with using tuning software. Good luck though.

antfarmer2

Bad or melted wire o2 sensor.

madmatster

Logging with Narrowband o2, using datamaster.

The only thing that changed was a new motor and cam . before i had no issues like this.

at WOT i'm at 11.64.

spark advance is at 29.2 during wot.. it sits there, and knock sensor doesn't look like its showing any knocking during the pulls.

BPW is between 8.3-8.7 during wot.

I'm new to this stuff. Just learning more going through this process.


I can spot the areas where the car bucks in the log. the TPS% drops from 100% to 60 to 0% within 2 records. it stays there for a few records, than back from 0% to 100% in the next frame.
Shouldn't the reading still be at 100% unless i lifted off the throttle? (regardless of what the engine is doing?)



i'm

Paccanini

Yes, it should.

What does your nb o2 voltage look like during WOT?

madmatster

O2 volts is showing low 900's during wot. (between 900-920) during the duration of the WOT pull.

SpedRacr93

What distributor/coil are you running in your '91? Stock HEI?

I would start by ohming all of your plug wires and then checking the boots and wires for any breaks/burns.

In my experience a good running engine with a bad buck under load is ignition related.

I know you said you gapped the plugs but it may be worth double checking them for breaks in the ceramic as well.

Before I swapped to an LS computer, I had issues with the HEI offering reliable service. I had the exact same symptom as you at one point that was solved by replacing the coil, though I was running a small HEI and external coil. It's hard to diagnose over the internet, but if I were you, I would be looking at ignition.

madmatster

Im running a D.U.I distributor/coil. I haven't ohmed my wires, but its easy enough to test them. The plugs are ngk6 and they are just 3 weeks old. i know for sure they are good.

the fact that it bucks LESS when gapped at .50 and MORE gapped at .35 might read as something ignition related.

Buccaneer

OK, you can not datalog or tune properly with a narrow band O2, it does not have the resolution needed like a WB O2. So, IMO, the data collected is not that accurate unfortunately. Honestly, you need a heated WB O2 to do this correctly without chasing your tail. 29* total timing? Something wrong there, knock sensor may be pulling timing. What do you have it set to in the software? My 383 likes it at 36*.

SpedRacr93

Trying to tune through the narrow band O2s is not a good method, though I'm not convinced that a slow timing would necessarily cause knock rather than just straight poor performance. I'm not familiar with the Datamaster but I'm assuming you can log Knock Retard at the same time as you're logging everything else? Just FYI - I always log Knock Retard when I'm recording data. It's the most detrimental thing and is something you want to be aware of when it happens so you can decide why it happened.

Throwing parts at it could get expensive quick.

I would test the coil and the module as well. Also, since you mentioned washing the engine, pull the cap and make sure you didn't get moisture and subsequent corrosion inside.

You do seem to be running rather rich. What is your AFR at idle? Do you have soot all over the rear fascia near the muffler tips? I hear what you're saying on the fact that the plugs are new, but they don't take that long to foul, either. I wish I could help you more with the tune, but I use EFI Live and my ECM is an LS1B.

Good Luck

Kevova

This would be a long shot you could call around and see if you find a shop with a "smart scope". Back in the 80s several companies built o scopes that would also interface with ecm. Manufactures put diagnostic functions in them, not concerned about that part. You can use scope function to watch secondary ignition and also watch all ecm data in real time. They can also have 4 gas so you can watch emissions. Most tech didn't know how to use them, but a shop with old timers may still have one. It could easily be an ignition issue.

Paccanini

Madmatster sent me one of his Datamaster log file and here is a screenshot of one the WOT pulls (red line = TPS-%). I assume it's done with 3rd gear:



As you can see, knock counts (blue line) increase from 171 to 182 between 2400 and 3400 rpm (black line). Mat, is this the rpm range where you sense bucking? Although ecm sees knocking, timing isn't pulled down. I wonder if the tuner has zeroed out the knock retard table in Power Enrichment mode. This can be done, if you suspect a false knock. Too much timing can cause knocking, but while knock count increases in the log, total spark advance is only 20 degree (pink line). Timing shoudn't be the problem? As many have said before, tuning is difficult without WB. Anyway, narrow band is showing 898 mV, when knocking begins. I would richen up the tune at that point and see if it helps any.

Another thing I didn't see any of this from the log file you sent me:

If you have another log where this occurs, can you send that file to me too? If you are able to do more WOT runs with logging, do it with 3rd gear from 2000 rpm till the end. That way we can see is the knocking always there. Remember to avoid the cops.

GREGGPENN

Put a volt-ohm meter on your TPS (engine off) and see what it reads moving from closed to wide open. SEE IF YOU SEE VOLTAGE BLIPS -- THAT AREN'T LINEAR!!! This was suggested early on...and seems of high priority with the blip in TPS% during WOT. FWIW...Voltage should read around .5v at idle...up to several volts when wide open. The change (increase) will be linear as you open the throttle blades (TPS).

900-920 O2 readings tells you you're not lean during your runs -- which is good. Considering you want enrichment, I think you're fairly close to the correct AFR -- per the tune you've paid for.

A steady advance is the weird thing to me...but I don't have a MAP system. My 89 is MAF and the spark stays dynamic (moves) during WOT operation. I'm wondering if it's related to your TPS dropping out? I'm betting TPS% plays an active role in calculations for advance...and when it goes to zero, the formulas "break"....which means it can't figure out target advance.

madmatster

thanks alot for analyzing this.

I feel like my bucking happens higher in the rpm range.

On the last pull on the log, record 6580 has me at 100% wot, then at records 6582-6585 i'm at 0%, then by record 6586, i'm at 100% again. (sensing i went from 0-100% in one record) like i lifted, but i def did not lift.
this happened on the 3rd pull too.. at record 5327

something is whackado with that i think.

I am going to do some more logging this weekend, and try to get a sense exactly where in the rpm range i'm getting my bucking.



My pipes are dusty and darkened after one ride. not too terrible, but def a bit dark.
I plan on pulling the cap and checking wires this weekend.

my afr is 14.7 at idle

i'm def at .5 during idle, but i never get over 4.0 at wot. Voltage looks gradual, for the most part.. except when it drops to 0 and then back to 100 in just a couple records timing.. (when im fully on the gas!). I am going to check voltage through the range this weekend.


my gut is telling me this is where my problem is. that sensor should read high voltage the entire time i have my foot on the pedal, regardless of anything else going on. right?

thanks for all the suggestions. i have some hope and direction.