FNG Needs Advice 1995 Tahoe Driveability

[Razzy1]

Hi there all! Long time reader here and I have found this forum very helpful, I just do not have any worthwhile experience or knowledge to add so first time posting. I want to thank everyone in advance for any guidance. Now to my issue:

Vehicle:
1995 Tahoe 2dr 4wd 350 TBI

Issue:
Stumble just off idle or when getting off the gas pedal engine will stumble a bit before returning to proper idle. What strikes me as odd is the truck ran great then one day the issue appeared. What I observe in the engine compartment is that when I barely open the throttle, the EGR valve steadily opens to 100% and the engine stumbles harder as valve opens. This is with throttle barely pushed open and held steady. I am not experienced to know how exactly the EGR system should operate but from TunerPro logs I see EGR being apparently always being commanded to 100%. I am at a loss as to if this is how the system should be operating or if I am chasing the wrong issue.

What I have checked:
New O2 Sensor
New ECT Sensor
Swapped EGR Solonoid and Valve
Checked all vacuum lines and replaced any questionable ones
Replaced several deteriorated Weather Pack connectors on various sensors
Swapped ECM
New fuel filter
New plugs, wires, cap and rotor

What strikes me as odd is my understanding is EGR operation should vary the amount is should open and not be either 100% open or 100% closed. Attached screenshot from Tunerpro capture. Bottom two lines in graph are throttle position and TPS voltage for reference. Middle green line is MAP sensor reading and the blue graph that spikes up to 100% and down to 0 is Desired EGR operation.

Does any of this look right? I thank everyone in advance. This old truck was my mother's baby till she passed so keeping it running means a great deal for me. Now its become my favorite toy

-Nick

 

[retorq]

I would put the notebook on mine and log for comparison but I had my EGR tuned out with the last chip.

 

[Razzy1]

Yeah, I wish I could just do that! Too bad I have to deal with California smog testing. That's what has me in a bind right now. Registration just expired and am pretty sure it won't pass smog with EGR or whatever the issue is not working right. At least judging by the exhaust smell...

 

[SunlitComet]

is this a port egr or negative back pressure egr?

 

[Razzy1]

is this a port egr or negative back pressure egr?

Hi there!

It is a negative backpressure EGR. The only real reason I am stuck on thinking EGR is issue is problem goes away if I disconnect it. Last night I managed to scrounge up one example of a TunerPro log that showed EGR operation for a totally different vehicle, and that car at least showed the EGR opening to varying amounts rather than just 100% open immediately like mine. I just for the life of me cannot figure out WHAT would be causing the system to think EGR needs to be open 100%.

Thanks, truly appreciate it.

 

[SunlitComet]

something is not right. first off your map reading should not be up and down like that unless something is the rpm is changing or something is bleeding the vacuum some how. and if it is a negative backpressure EGR the vcm cant actually measure linear movement so the egr reading you recorded would have to be the solenoid commanded on and off. now if it opens and closes as the solenoid does there would likely have to be a clog in the backpressure vent thru the egr stem that keeps the vacuum from holding the diaphragm upon against the spring the whole time cause as long vacuum is present at port of egr, backpressure that is bled thru against the vacuum should control smoothly the egr movement. either one of the two springs is broken, valve diaphragm is damaged or backpressure bleed port is plugged. the only other thing that comes to mind is that you have an engine vacuum leak throwing off the map sensor or it is defective and/or you have an exhaust issue that is effecting backpressure and therefor proper control of egr.

 

[Razzy1]

Thanks for the interest, really!

Exhaust issue:

I hope not, entire system from cat to muffler replaced ~3000 miles ago as when I brought it from Tennesee the system had no cat and muffler had for sure suffered at the hands of years of road salt and snow, lol.

clog in the backpressure vent thru the egr stem" or"one of the two springs is broken, valve diaphragm is damaged or backpressure bleed port is plugged

I am "assuming" these are parts/sections within EGR valve itself? Both EGR valves I have behave exactly the same as well as all 3 solonoids I have tried. Manually pushing EGR valve or adding vacuum to cause EGR to open produces stumble so I am assuming EGR port is not plugged. Ohh and yes, EGR operation does appear smooth and steady, its just it seems its only operation is smooth and steady to 100%. But perhaps my ignorance of how this system should work is finding problems where there are none? Maybe it should be 100% open at part throttle?

the egr reading you recorded would have to be the solenoid commanded on and off

Exactly, just seems REAL off that all I see is just a straight 100% open command till I release throttle, unless software is not capable of showing the actual command pulses on/off that actually regulate amount of EGR. However I can tell ya from watching the EGR, it actually IS opening 100% as soon as ANY throttle input is given and keeps opening till engine stumbles harder the more it opens.

I will check once again for vacuum issues. I am pretty sure the few vacuum lines that exist on this engine are ok as that' was one of my first checks. Plus I've sprayed almost a whole can of carb cleaner around throttle body, brake booster, vacuum lines and fittings checking for leaks I may have missed. Will go through a more proper test with vacuum tester I suppose.

Again appreciate the advice. Have always been good at the remove/replace type repairs and would rather learn myself than just throw money at the stealership, lol.

 

[SunlitComet]

verify the proper routing of houses to the correct ports on the throttle body as well. let me know what you find.

 

[Razzy1]

Yep hose routing is just like in your attachment. Have triple checked that both with the label in engine compartment as well as the GM factory service manual set I recently bought.

I have yet to check the timing, need to get a timing light. I don't know if timing being off could cause stumble, causing pressure to rise, causing MAP to think EGR needs to be all the way open. Sadly that line of reasoning could be pretty stupid I admit, lol.

Only possible leak I hadn't really checked until just now was brake booster, I pulled the line from it an plugged it to just make sure there couldn't be an internal leak inside the booster. No change.

Also just did a run through of the "EGR System Check with Negative Backpressure EGR Valve" from the 1995 Service Manual and every step of the diagnonstic flow-chart tested perfect, eg. holding vacuum, solenoid operation, etc. Pretty much confirms what I had already figured regarding the EGR system itself, just didn't have a vacuum pump to do the proper flow-chart run-through.

Thanks again,
Nick

Edit:
Forgot to respond to one other item from previous post. As per the MAP reading going up and down on the chart, MAP on that chart is starting to rise as soon as I give a little throttle and hold it steady slightly depressed. MAP reading continues to rise and well as EGR valve continues to open till it hits 100%, all while holding the throttle steady at slightly open. Hope that clears up what I am seeing. I will admit TunerPro info is over my head quite a bit and am still getting a handle on it. However seemed a better investment to buy a $70 cable that I could hook to my laptop rather than buying an expensive Live Scanner to do the same thing.

 

[Razzy1]

Wanted to post an update should this be useful to others:

So after swapping through one parts store replacement EGR, my original EGR, plus one borrowed spare, all same part # I had reached the point of wanting to just kill this thing. On a whim I ordered an AC Delco EGR of Amazon with what research appeared to be an updated part for my model/year. Basically I figured it would not work and Amazon has easy returns. However I kept reading how picky these TBI trucks are over EGR's. Well dropped in new EGR and truck runs better than ever in the couple years I've had it!

Lesson to be learned here:
These trucks for sure ARE picky about EGR's! Every other EGR I threw in it including original were tested multiple times to the exact steps in the GM Factory Service Manual for my truck and always tested perfect (according to the flow-chart.) However obviously it was still a bad part.

SunlitComet: Thank you for attempting to help! Truly appreciated!