Assistance req'd dignosing data

[decosse]

94 Cadillac STS L37 Northstar

I'm trying to diagnose a problem with high HC (1000 count) at idle - there is no problem at 2500 rpm, is only 1 count.

So far, I have replaced the FPR, injector seals (seats & stems) & a leaky o-ring at the main fuel-rail connection (this may have been damaged during install of other components however); also have replaced both O2 sensors.

Please see a picture of the O2 sensor responses from a GM TECH 2 engine analyzer tool.

It looks to me like the front sensor is showing a much different scenario from the rear one, in that it is trying to trim but going to extremes, vs the rear which looks more in a controlling mode. (When you raise rpm to 2500 or so, both amplitudes & duty cycles appear more similar in a "controlling" region.

Am I analyzing this correctly, in that there appears to be an issue with the front bank? I have switched all the injectors front to rear & it made no difference. I was hoping to move the problem from front to back & identify (or eliminate) an injector problem.

The bank 1 fuel trim is adjusting to the lower counts - especially as the A/C kicks on, it will drop to 108 counts, while rear stays ~ 128

Any suggestions based on the data plots as to what else I should looking for? Any other data I should check to help pinpoint the trouble?

Appreciate any assistance!

[KevinW]

Sounds like you've ruled-out the injectors, however, there's still the issue of the injector drivers. It's possible a driver problem is causing a misfire in one of the banks; I would guess the front bank since the BLM drops as low as 108, and swapping injectors had no effect.

[cpk]

I'm taking a wild stab here, as what little I know I learned from various boards, but with the 4.x Cadillac engines, it was possible to disable each injector individually at idle. The idle would change for all the good injectors, but a bad injector would not alter the idle. Even if you swapped the injectors, if you had an electrical issue in the wire harness, it still wouldn't be working properly. Can you do that on the N*?

Also, doesn't the N* have an EGR system? Could that be malfunctioning and affecting the exhaust?

Just tossing out some ideas...

[Logan]

Need more info....

Are the O2 sensors new GM ones...or aftermarket?

I understand 'front' and 'rear' O2 sensors...

What may be confusing me.....is...the bank 1 and bank 2 fuel trims.. I understand the 128 etc but..

Bank1 on a Northstar is the rear bank.

You mention a data change with the AC on....have you checked the clutch diode?

Can you offer more details?

[decosse]

Your assistance is greatly appreciated - I'm getting stumped!

Are the O2 sensors new GM ones...or aftermarket?

After-market - had the same signature traces before & after replacing them however

What may be confusing me is...the bank 1 and bank 2 fuel trims.. I understand the 128 etc but Bank1 on a Northstar is the rear bank.

Now I'm confused - my assumption was the one that was trying to trim the fuel (the bank 1 trim) was the front since the front o2 sensor is the one with the odd trace (or is the correct one???) Which trace looks incorrect to you - the front or rear sensor?

I have no doubt you are correct given your experience but that makes me even more puzzled now to understand the data.

(why can't they just use the same reference for both! i.e front & rear or bank 1 & bank 2, but not a mixture!)

You mention a data change with the AC on....have you checked the clutch diode?

The big question is why does only one bank adjust the trim in this condition - whther the one that's adjusting the trim for the additional load is the "good" one or the "bad" one???? What is your thought process here - the diode is pulling current & affecting power to the PCM?

I think the symptoms of the fuel trim & the O2 sensor must be related - one O2 sensor different from the other & one trim "working" while the other is really not doing much as the load changes.

Can you offer more details?

145K miles

Car failed CA smog for high (1000 count) HC at idle only - is 1 at 2500rpm.

All other paramters were in spec i.e. negligible counts

In the last month:

New Fuel Pressure regulator - new injector seal kit; replaced o-ring from feedthrough to fuel rail; all injectors switched out front to rear; fuel pressure is solid at 40 psi & holding tight with motor off; new O2 sensors front & rear;

new fuel pump (had failed just week before I got into the SMOG and started all the dignostics - coincidence?);

Water Pump.

Recent history (last 9 months or so)

Replaced all plugs (OEM type) and new plug wires; one bad coil replaced;

Cleaned phenoilic spacer & manifold - new gaskets; cleaned EGR;

What other data or info would you like to help you help me?

[Logan]

Ok....I went and tried my Tech2 for comparason. Mind you its a 2001 Northstar....so heated O2 sensors and a different computer sytem.

Both O2 sensors on mine look like your front O2 sensor data. Wide sweeps.

Your rear O2 sensor data has strange sharp peaks at the top of each voltage swing.

Your bank 1....which is the rear bank of the engine.....is where the fuel trim is being pulled back.

So it relly looks as if the problem is in the back side.

And now I have a question????? I just got my new 32mb card for the Tech2. Wheres the 'live plot' data located?

[decosse]

That is a HUGE help Logan- now makes a little more sense! & also changes the whole picture of what's going on.

So, still to find the cause - I'm starting to look at ignition again - one of plug wires (#3) has a poor fit on the coil - I ignored it because I was focussed on the front!! So I'm going to replace that and also check out the coils - I'm going to rotate in a new one through the different positions to see if any different. It had a slight misfire going up the long haul hill on my commute - very similar to the problem I had when coil ewas failing; might just be the wire though!

To your Tech 2 question - did you mean to download it off the Tech 2? (Since you apprently saw the live plot)

[KevinW]

OK, so the 108 BLM corresponds to the rear bank. It looks like an intermittent misfire is biasing the O2 sensor voltage low, as shown in that plot. Has a compression/leak-down test been performed? I assume the ignition components have been ruled-out. There could be a mechanical or electrical reason for a misfire at low engine speeds. Do you have a scope to check the drivers?

[Logan]

Yeah...I saw the 'live plot' on the Tech2 screen.

Someone told me about a year ago they could watch 'live' data in the plot mode. Like a scope. I've never seen it. Thats why I was asking if thats what was shown.

To plot a map like that on mine. I have to record the data....then select the sensors I want to plot.....then display the recording in that mode aka 'snapshot plot'

So if you look at the top of ours....it displays 'snapshot plot'.....PLUS...it displays our O2 sensors..

Let me know...maybe Vetronix has some dealer software they are not telling us about

[decosse]

OK, I see what you mean now Logan - I thought you must have been in live mode originally.

You just select the two sensors to display then accept, then select the "more" function on the right & it will bring up the "live plot" option.

Once you get into live-plot mode another nice feature is you can "zoom in" or "zoom out" to change the timebase to expand the signal so you can see the detail between pulses.

As to the problem: now that I was re-focussed into pursuing the rear bank, wanted to follow my hunch on the ignition again; Disconnecting the plug wire from #3 first & then re-connecting & comparing to #5, there was a HUGE difference in spark intensity as it was arcing - weak coil maybe? (although still pretty intense spark, just nothing like the other one); so replaced the coil with new one which had even more intensity! However no difference.

I was sure I could hear a very rapid clicking however & started looking for arcing - very difficult to spot, even in dark but then found that the whole plug wire harness to that rear bank was arcing with a faint purple corona all around it, with a intermaiitant more visible snap from different locations. So much for a new GM set of plug wires less than a year ago!

So I have yet to vailidate with replacement but it's looking pretty much like that's the issue.

[Logan]

Ok...check the wires on rear bank....

As to the Tech2... I know some of the steps you are talking about. I can look at the complete 'movie' etc of the data. I can zoom etc...but I can only see recorded movie data/snapshots.

So.. the question I have...was your data 'live' or was it a recorded 'movie.'

If it was 'live' data....like a scope would see....I'm calling Vetronix in the morning.

Get ready...Vetronix has the next system to drain our money supply. The CAN system. Controller Area Network is the next protocal. Its used now on Saturn IONs and a few other cars.

[decosse]

Definitely Live - (if you look at my pic, you can see it says "live plot" right on the screen vs yours which says snapshot plot.). As well as the zoom functions, it also has a "Pause" feature - you can see on my screenshot it has a "Resume" icon - that is because I paused it for a static shot; "resume" obviously re-starts the data stream.

I think this is a std feature of the tech 2 - yours looks possibly a little newer than the one I was using but essentially the same - 32MB Tech 2.

What mode are you in when you select the parameters to plot? This was from the engine diagnostics mode.

Live plot isn't immediately available listed option on the screen after you first "accept" the parameters - you need the "more" selection to bring that feature up - select the "more" tab on the right after you "accept" the selected paramters to display, then that option should be available on the next view.

Unfortunately I'm not looking at it right now, so can't give step/step direction, but it was pretty intuitive - I only used the thing for first time a week or so ago.

[Logan]

OK! Found the live plot data mode on the Tech2. I was looking in the wrong menu.

I dont think the 10mb card had that feature.

Thanks

[Logan]

As for our Tech2...its actually one of the older HP made models. The current Tech2s are made by Vetronix. Bosch recently bought Vetronix.

[decosse]

Any recommendations on a brand of ignition wires? Reluctant to go with the GM part again ......

[Dloch]

OK, so the 108 BLM corresponds to the rear bank. It looks like an intermittent misfire is biasing the O2 sensor voltage low, as shown in that plot. Has a compression/leak-down test been performed? I assume the ignition components have been ruled-out. There could be a mechanical or electrical reason for a misfire at low engine speeds. Do you have a scope to check the drivers?

An intermittent misfire will certainly bias the O2 sensor voltage which will move the BLM which in turn will lean out all the injectors on that bank.

Dennis

[decosse]

Well I'm not impressed - bought a new set of leads today - Standard brand: although not nearly as bad as the current ones, there is still some arcing from these. I put the old ones back on & these are gong back to the store tomorrow!

Anyone got good experience of the best quality lead? Brands I can find specific to Northstar fit are OEM (AC Delco); Bosch and Autolite (Professional & Pro-fit varietals)

I'm almost tempted to go with a generic fit high performance 8 cylinder set that have a better insulation capabilty.

[cadillacjeff7777]

I wouldn't suggest Autozone. Here in Michigan the only size that Autozone has is 7MM. Not like you really would want anything autozone on your cad but just stating that they even have screwed up the wiresets.

Anyway, I wouln't be too turned off by the wires from the dealer. That is my choice for purchasing wires. Maybe try a NAPA ir Pepboys...Just make sure that they are atleast 8MM.

[Dloch]

Well I'm not impressed - bought a new set of leads today - Standard brand: although not nearly as bad as the current ones, there is still some arcing from these. I put the old ones back on & these are gong back to the store tomorrow!

Anyone got good experience of the best quality lead? Brands I can find specific to Northstar fit are OEM (AC Delco); Bosch and Autolite (Professional & Pro-fit varietals)

I'm almost tempted to go with a generic fit high performance 8 cylinder set that have a better insulation capabilty.

Did you buy the AC Delco Brand? I just purchased a set of those, PN-748J I believe and new set of 41-950 plugs both of which set me back almost $ 190.00 at the local auto discount store. I believe they were 7MM.

In any event I haven't seen any adverse effects although I haven't looked under the hood for a light show at night yet.

Dennis