: Engine Data Analysis help



decosse
02-15-04, 05:19 PM
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 the outside 2 injectors front & rear & it made no difference, getting ready to switch the middle two: I'm hoping to move the problem from front to back & identify (or eliminate) an injector problem.
Am I on the right track?
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!

growe3
02-15-04, 07:10 PM
You will need to post what year, model and engine you have, for anyone to assist you.

-George

decosse
02-15-04, 08:22 PM
You will need to post what year, model and engine you have, for anyone to assist you.
-GeorgeI'm not sure what difference it would make actually, we're talking pretty generic diagnosis on the data supplied - the question could be related to V6 or V8 & not necessarily even a a GM (although it is) but happy to edit the initial post to accommodate. Thanks for your suggestion.

BeelzeBob
02-16-04, 11:53 AM
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 the outside 2 injectors front & rear & it made no difference, getting ready to switch the middle two: I'm hoping to move the problem from front to back & identify (or eliminate) an injector problem.
Am I on the right track?
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!


I think you are on the right track with your analysis and logic. More than one way to skin a cat but your approach will help isolate the problem to an injector or not.

Have you had the intake off and cleaned the phenolic spacer plates and the "mouse holes" machined into each of the intake ports..?? Although the EGR system is not enable at idle there is some level of "crosstalk" between the intake runners thru the intake port mouse holes and EGR passages. If the EGR passages are not clean and/or the mouse holes are not clear then it could possibly cause a slight fueling imbalance on one side.

It doesn't take much fueling imbalance at idle to screw things up as the amount of fuel being delivered is very small so any drip or leaking injector will have a large (percentage) impact.

If the engine has a lot of miles you might also consider a compression check and/or leak down check of the cylinders to compare. If a valve is not seating correctly due to valve guide wear and/or carbon buildup on the seat of the valve then it could also cause a fueling imbalance due to the cylinder volumetric efficiency loss....

decosse
02-16-04, 02:01 PM
If the engine has a lot of miles you might also consider a compression check and/or leak down check of the cylinders to compare. If a valve is not seating correctly due to valve guide wear and/or carbon buildup on the seat of the valve then it could also cause a fueling imbalance due to the cylinder volumetric efficiency loss....That's what I'm worried about I guess - car has 145K miles. Thanks for that suggestion & the others:

Swapping all the injectors front to back made no difference, so looks like injectors eliminated. The Fuel pressure is now holding pretty tight at ~40psi.

I had completely cleaned the phenolic spacer & "mouseholes" ~ 9 months or so back.

New plugs ~ 9 months ago as well as new leads and one replacement coil. I might replace plugs again just for elimination - I am discounting coils since each one is common to a front & rear cylinder - I think I can switch the wires on each common coil, right? (i.e. switch 6 & 7 connections) to see if one half of coil is marginal?

Other clues -
#8 plug looks a little rich vs the others; (Don't really understand that - from a fuel standpoint all fronts should be seeing same fuel - potential ignition problem? or cylinder/valve per your suggestion)
When the A/C kicks on (i.e. more load) the short term trim at idle on that bank goes really low ~108 counts, vs mid 120's or so when off (the rear bank stays pretty much rock solid at 128)
It hunts a little at idle - not really terribly rough, but noticable.

BeelzeBob
02-16-04, 04:18 PM
That's what I'm worried about I guess - car has 145K miles. Thanks for that suggestion & the others:

Swapping all the injectors front to back made no difference, so looks like injectors eliminated. The Fuel pressure is now holding pretty tight at ~40psi.

I had completely cleaned the phenolic spacer & "mouseholes" ~ 9 months or so back.

New plugs ~ 9 months ago as well as new leads and one replacement coil. I might replace plugs again just for elimination - I am discounting coils since each one is common to a front & rear cylinder - I think I can switch the wires on each common coil, right? (i.e. switch 6 & 7 connections) to see if one half of coil is marginal?

Other clues -
#8 plug looks a little rich vs the others; (Don't really understand that - from a fuel standpoint all fronts should be seeing same fuel - potential ignition problem? or cylinder/valve per your suggestion)
When the A/C kicks on (i.e. more load) the short term trim at idle on that bank goes really low ~108 counts, vs mid 120's or so when off (the rear bank stays pretty much rock solid at 128)
It hunts a little at idle - not really terribly rough, but noticable.


With this many miles is it causing an emission in use compliance issue or just an idle roughness complaint or ???????

You might take the top cover off the intake and make sure that all the tuning tubes are tight and in position and sealed at the port end. If any of the mounting/locating screws are loose or gaskets loose/missing the tuning tube will not be effective and will "leak" at the tube to port interface inside the manifold. This is a "tuning" leak only but will cause one cylinder to act funny sometimes. Not a common problem but could be a possibility on a high mileage engine.

You might also start checking the front bank exhaust manifold carefully for an exhuast leak. If there is a leak it can cause the O2 to read the air that is sucked in at the leak during the negative pressure pulses in the exhaust manifold. The system will then react and richen the mixture.

Try also plugging the evaporative emissions canister line so that there is no purge flow from the canister. Possibly the canister is saturated with fuel/heavy ends of HC and is contributing a large portion of the fuel at idle which could cause a maldistribution and high HC on some cylinders at low thruput..i.e...idle.

Did you run the engine good and hard and get it good and hot for a long stretch before undertaking the measurements each time?? If it is short tripped a lot and then you start into loooking at the O2 activity and such then it could still be recovering from the cold start/short trips previously. Take it out and drive it and clean it out good before procedeing if not earlier.

BeelzeBob
02-16-04, 04:21 PM
Also, maybe you mentioned this earlier but have you tried swapping the O2 sensors and/or replacing the front one?? I always hate to trouble shoot with new parts swapping but along the lines of the injector swapping front to rear the next thing to do would be the O2 sensors swapped front to rear....but they are such a pain that a new one might be easier in the front to see if it clears up the problem. Possibly there is some contamination or other problem with the O2...???

decosse
02-16-04, 06:19 PM
Also, maybe you mentioned this earlier but have you tried swapping the O2 sensors and/or replacing the front one?? I always hate to trouble shoot with new parts swapping but along the lines of the injector swapping front to rear the next thing to do would be the O2 sensors swapped front to rear....but they are such a pain that a new one might be easier in the front to see if it clears up the problem. Possibly there is some contamination or other problem with the O2...???Thanks for your continued advice - I'm running out of ideas myself!
It failed CA smog test - only the high (1000) HC at idle - all other parameters well within spec.
Both O2's were replaced - first thing I did!
The exhaust leak worth considering - I had a leaking water pump which was dripping down in that front O2 region - possibly could have caused a fracture? Hmm...



Try also plugging the evaporative emissions canister line so that there is no purge flow from the canister. Possibly the canister is saturated with fuel/heavy ends of HC and is contributing a large portion of the fuel at idle which could cause a maldistribution and high HC on some cylinders at low thruput..i.e...idle.This is new territory for me - not even sure where the canister is! How/where should I plug it?

BeelzeBob
02-16-04, 11:02 PM
Find the evap canister purge solenoid on the right hand cam cover I believe...and follow the vaccum line to the throttle body. Disconnect and plug the canister purge line to see if that makes any difference.

decosse
02-18-04, 01:34 AM
Find the evap canister purge solenoid on the right hand cam cover I believe...and follow the vaccum line to the throttle body. Disconnect and plug the canister purge line to see if that makes any difference.No difference.
It looks like problem is in the ignition wires to rear bank - that whole set of wires to the rear bank has corona & crackling going on - I'll confirm after replacement for the knowledge archive.