jacked1

Injuneer,
Attached is a data log generated according to the above guidelines.
Thx, Rick

Injuneer

I've reviewed heavily modified tunes. The logs are not slow.

Injuneer

Scan is messed up. Log starts at 13 seconds engine run time (OK), seems normal up to 106 second run time, then a strange 1542 second reading (probably interference on data line - not unusual), then there is a blank line of data (????), and the next line starts at 319 seconds run time. 3.55 minutes of data missing. What happened?

Scan rate looks really good (5 to 9 frames/sec) for the first 442 seconds, then is craters and drops to 3 frames/sec, even as low as 0.4 frames/sec.

I can review the closed loop operation.

@ GaryDoug

jacked1

Injuneer,

Thx again for taking a look. I think the laptop went to “Sleep” while the 5 minute idle from a cold start was being logged. I thought that the screen display just timed out but now since data is missing I’ll bet the machine went to sleep. I woke it back up not even realizing the laptop went into sleep mode. If needed I can run another log using the guidelines you posted for me a couple days ago. Just LMK and I will run/post another.

Thx again, Rick

Injuneer

I'll look at what we have of the closed loop operation.

Injuneer

You still have a problem with the IAC. It's bottomed out at "0" counts and the engine is idling unsteady, at 140-290 higher than it is supposed to. 790-940 RPM vs. 650 RPM target)

There's something wrong with the fuel on Bank 1. At idle the PCM is using the LTFT's to add fuel, ranging from 23% extra fuel at idle to a range of +9% to +13% in the other operating cells. Bank 2 is showing a fuel addition in closed loop, but it's not as large as Bank 1, mainly +8% at idle, On the other hand, vacuum is not lower than it should be. A slight contradiction. Vacuum is fairly steady at idle, at 20.7"Hg +/-0.8" Hg. MAF is elevated, but that's because the idle is elevated, but may still not be seeing all the air entering the engine.

It is definitely running lean, because when you hit the throttle hard (88% max, never reaches 100% where it should be when it's "on the floor") and the PCM goes into power enrichment mode, the Bank 1 O2 sensor drops at times and you get a max 10% knock retard. You haven't driven much in Cell 15, so the LTFT's are 129 on Bank 1 and 128 on Bank 2, vs. the large additions on Bank 1 in the more used Cells.

The engine is getting air from somewhere other than the IAC system.

When you were driving for this data log, did you see "rough idle/backfiring and shuddering under acceleration"?

I still need to look at what the EGR and EVAP systems are doing while driving. I'm also puzzled why the IAC was reading so high in the second data log you posted.

jacked1

Hi - yes while running this recent log I still was getting shuttering under both hard acceleration and normal acceleration. The backfiring was happening when my foot was off that gas and coming to a stop and also while idling once the engine was warmed up and running in closed loop.

Thx, Rick

jacked1

Injuneer,

When you mentioned checking the EGR numbers in the log file it reminded me of something I had read on how to test it.
Last night when the car was warmed up I manually pressed in the diaphragm in the EGR valve and the engine RPM didn’t change a bit. Would this be a legit test showing that the EGR valve is bad possibly? From what I’ve been reading if the EGR is working, manually pressing the diaphragm while the car is warmed up will almost cause the engine to die out or cause it to immediately run badly. I could always pull it and check it out to see if it holds vacuum or in need of being cleaned. This little issue is driving me a up a wall - as if it’s not evident in my posts. LOL..


Thx again for your help. Rick

Injuneer

Should cause it to stumble and possibly stall. If you have a vacuum pump, check the operation by pulling a vacuum on the line from the solenoid to the valve, to open the valve and check for the stumble/stall.

Now..... if it's plugged up and not flowing any EGR, it's going to have very little impact on the way the engine runs, whether it's supposed to be open or closed. A no-flow situation should set DTC 32, but the OBD-1 diagnostic for the EGR is not very sensitive. Many people have deleted the EGR valve and never had a code. On the other hand, if its "stuck open" and has been passing a SMALL amount of exhaust all the time, it might account for the rough idle and stumbling issues. EGR is not used at idle, not used at WOT, and is only used below 3,600 RPM. Mainly used at lower RPM while "cruising".

jacked1

Ok - something I completely forgot about. When I got the car 18 months ago the air pump was hooked up to power but all hoses were disconnected. It was just sitting there making noise so i removed it. I’m admittedly ignorant when it comes to how the air pump and EGR valve work together but I’m guessing the air pump in this case pumps air into the exhaust manifolds? The EGR lets a bit of exhaust gas’s into the intake and combustion chambers to ensure a cleaner burn and lowers operating temps and emissions. Not sure how the air pump works in conjunction with the EGR valve or what impact they have on each other.
I’m thinking your are correct when you suggest they EGR is stuck open a bit causing this issue (vacuum leak) if I’m understanding correctly. I think my choices would be remove and examine the EGR valve and clean or replace it or even remove the EGR and just get the blocking plates and caps to cap off the intake manifold outlets going to the EGR.

Is my reasoning solid here?
With you knowledge base behind you what would you recommend - getting rid of the EGR or replace it if does prove to be the cause of the vacuum leak? I plan on continuing to use the z as my daily driver for the warm months with limited use during the winter time.

And once again - thx for all your time and input here.
Rick

Injuneer

Easy one - AIR and EGR have nothing to do with each other. The only minor exception would be the 1993 model, where a small portion of the AIR piping and the EGR piping share a common pipe. But even that doesn’t mean the systems are interconnected, because only one system operates at a time.

The AIR pump only runs on startup, and only for a few minutes. The cold engine starts with a very rich A/F mixture, and the cats are cold and don’t start working until they reach a temperature of ~600°F. The air injected into the exhaust manifolds helps burn any excess fuel (unburned hydrocarbons) reducing pollution, and the heat generated by this combustion helps heat the cats up faster, reducing pollution. AIR pump shuts off, and is not used again until the next engine start.

EGR can be used any time in closed loop, except at idle, at WOT, or above ~3,600 RPM. It is only used, in controlled quantities when the PCM sees (programmed) conditions the may cause combustion chamber temperatures to elevate to the point where nitrogen (78% of air) starts to combine with oxygen, forming the pollutant “oxides of nitrogen”, or NOx. Typical example is lugging the engine while attempting to accelerate in an overdrive gear when the engine is operating at low RPM.

Since there is no interaction or physical interconnection (1994-1997) issues with the AIR system cannot cause a vacuum leak via the EGR valve. However, a leak at the AIR connection on the exhaust manifold can cause the exhaust to induct air into the exhaust, throwing the O2 sensor off, causing a false lean condition = elevated LTFT's and excessively rich (fouled plugs) conditions on that bank of the engine.

It would be worth checking the AIR connection points on the exhaust manifolds, particularly Bank 1 for leaks.

The only way the EGR system would cause a lean condition would be if the gasket for the EGR valve was bad (unlikely) producing a leak all the time, or if the EGR riser tube or any connection was cracked, leaking, etc., producing a lean condition when the EGR system had he valve open. But because the EGR contents are distributed through the intake manifold, the lean condition would appear on both banks.

jacked1

Hi - thx for the detailed explanation. The car actually has long tube headers. I checked anyway and don't see any type of open AIR connections. The only thin going into the header tubes are the 02 sensors.
Just spent the day going over everything again. Checked all vacuum lines, intake leading to the throttle body, exhaust. Didn't find anything cracked or rotted out that could leak air into the system. Cleaned the IAC valve and still no real progress.

I'll pull the EGR valve this week sometime to see if I can see if that is the point of the leak. When inspected it looks tight and clean. I even took a long shot and tried a MAF sensor but that didnt help anything either. That will be going back.
I'll have to keep chasing it.
Thx man. Rick

Injuneer

I just realized I have 5 different people I'm simultaneously trying to help (4 here, 1 on another LT1 site) with data logs at the same time. Please be patient if I don't get back to you right away. These can be very time consuming.

jacked1

Hi Injuneer,
Helping 5 of us @the same time? That is a lot - thx a ton for all the help and insight you have provided so far. Now since my last post i have been working thru all possibilities on my 94 that i can come up with. I did discover that the EGR valve was unable to hold any vacuum @ all so it was creating a vacuum leak just by staying in the open position as far as I understand it. I replaced it and drive the car for about 15 minutes total yesterday and the symptoms seem to have gotten less in severely but the rough idle (ESP while in drive) remains as does the shuddering under under acceleration from a rolling start until the trans kicks down a gear and backfiring while idling warm and while cruising to a stop - foot not on the gas. Car also idles in neutral a bit high @ 1000-1100 rpm when warmed up. Will replacing the EGR valve require a period of driving in order for the computer to relearn for the new part? How about the TPS? I’m going to look @ that and see if I can find an issue.

I should probably run a new log. When you get a second let me know what you think - if i should drive the car for a bit more and see if the issues dissipate once the PCM acclimates to the new EGR valve and if there is anything else you see that i may be missing.

Thx,
Rick

Injuneer

The EGR valve is normally closed. The pintle is extended and blocking the port that allows exhaust gas to enter the intake manifold. If there is no vacuum to the diaphragm that retracts the pintle to open the valve, there is no movement of the pintle, and nothing (neither exhaust or air) can enter the intake manifold. There was no vacuum leak created by the lack of vacuum to the diaphragm.

High idle can only result from excess air entering the cylinders. If the IAC valve is operating correctly, and the PCM is telling it to completely close, the excess air is coming from somewhere else. The problem with the IAC is that there is no feedback to the PCM to tell the PCM is it's actually working. The IAC “counts” are a measure of how far the PCM is telling the valve to open. If the valve is stuck open, ore carbon buildup is preventing the pintle from seating/sealing, excess air could enter. Worth removing the IAC to see general condition. Do NOT attempt to move the pintle. It can be cleaned by lightly brushing with solvent, but don’t let the solvent get inside the valve. Limit contact to the pintle, with the pintle facing downward.

Shoebox shows how to check the electrical part of the IAC:

http://shbox.com/1/iac2.jpg

You TPS looks fine in the data log. Why do you suspect it as a cause of the problems?