94LC Failed CA smog no EGR vacuum when revving. (1 Viewer)

May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
Hi Folks,
241K miles now.

Prior to test:
Replaced cats last month. It was clogged and heating up the floor pretty bad.
Disabled Pair valve which was the cause of my Code 25 and 26. Stuck and corroded valve.
New O2's
New egr temp sensor
Dorman VSV
AC Delco charcoal canister
Morning of test I noticed my E and R ports were clogged again, cleaned.
Thought I was ready so off I went.

Comprehensive visual inspection PASS
Functional check FAIL( EGR no vacuum when revving)
Emmision test PASS (haha emission passed even with disabled PAIR)

After test======

EGR valve:
Engine dies when applying vacuum. Nothing clogged.
Removed valve, applied vacuum with hand pump while plugging opposite end and it holds vacuum.

Bad modulator:
Blew from bottom and heard air going through port R.
Replaced from Autozone for the same exact one, Denso, with a $10 discount - $84 not bad.

EGR temp sensor:
Verified both wires from ECM to harness have continuity.
Getting 4.95V at harness when IGN switched on.
Oil test from FSM..all temp range OK but in the 302 deg F range I was getting around 4.x K ohms.
Just a little above the 2 - 4K ohm range.
Installed a 3.3K Ohm resistor in place of the temp sensor, unplugged ecm harness and measured 3.28K Ohms at the ECM harness side.

Dorman VSV:
Resistance test ok. No short to body from terminals. Voltage applied and it clicks.
When voltage applied no air can pass.
I don't use the intake manifold pipe.
Bypassed this by routing hose outside of manifold direct to EGR valve as my VSV is zip tied to manifold.
Checked continuity for each of the wires from ECM to VSV harness.
When I direct connect EGR port on ECM to body ground VSV clicks.

Removed throttle body and upper intake chamber.
Clogged metal Y pipe that joins EGR vsv with an adjacent VSV, cleaned.
Clogged EGR passages of intake chamber, cleaned.
Just replaced all rubber hoses.
Ports E, P, and R have no vacuum at idle and have vacuum at 3500 rpm.

Circuits that I could see on the wiring diagram that are connected to EGR temp sensor are VAF, ECT, and TPS sensor via the Br-B wire.
Performed on vehicle resistance test of TPS and all checks out.
My VAF did leave me stranded due to a bent fuel pump switch in it but bent it back and works now. I have a thread on this somewhere.
Later found out that all of the 3 wires inside it were disconnected from the board, since I've had it, but had them soldered properly. Resistance test checks out too.

Per FSM only inputs to EGR is Batt(input 9) and ECT(input 1).
Battery is almost 7 yrs old.
Disconnected ECT harness and get 4.95v when IGN is on.
Measured resistance without removing ECT, both hot and cold and all resistance checks out.
After warming up engine, disconnected the ECM harness for the ECT.
Measured resistance and got .3K ohms. (FSM .2 - .4K ohms 176 degress F.)

Vacuum test:
Ports E and R are clean and have vacuum.
Vacuum from Port Q hose leading to EGR valve is good.
Perfomed the FSM test where you disconnect the hose leading to port R of modulator then connect a different hose to it and connect the other end to intake manifold. I connected it to the brake booster hose.
Since my VSV is not responding I plugged the other side of the EGR with my finger to simulate VSV working and got a really high vacuum when revving to 2500 rpm.

Vacuum test while driving:
T'd between EGR and VSV and routed the hose through firewall to my gauge.
Idle and WOT no vacuum.
Slight throttle got up to about 4 in/HG.

T'd between modulator and EGR valve, same results.

VSV Voltage test:
Disconnected VSV and ran some jumper wires from VSV harness to my DMM inside the truck.
When revving while parked the most I got was anywhere from .2x to .5x V.
I don't remember if it was the moment I shifted to D or the truck started moving that the voltage started increasing to 13.5x V!!!!!

Since everything checks out it leads me to replace the ECM.
But why will the VSV get proper voltage only when driving?
Only difference I could think of is it could be the VAF as the difference between being parked and driving is air flow, considering only possible circuits that are connected to the EGR.

Getting tired and impatient so bit the bullet and ordered a used ECM on ebay.
If it doesn't work I have a backup if nothing is wrong with it.

Appreciate any inputs you have.
Thanks.
 
Joined
Sep 15, 2016
Messages
696
Location
Chattanooga, TN
Nice work on the troubleshooting and reporting.

It's a long shot but what's the routing of the power wires for the EGR VSV switch? I assume that they go past the EGR hot pipe. Have you inspected those wires for shorting and insulated them/etc.? I've seen electrical shorts respond to a car going into drive because the drivetrain will shift slightly and cause a physical movement of the shorting wires. It's possible that the power source to the VSV EGR is shorting out or has a weak connection when the engine is resting but then moves off of the short or gains a better connection when the engine shifts into the drive position?

Not directly related to your problem but you may want to try to blow out all of the EGR vacuum lines if you haven't already. My E/R ports clogged a few times, as they were essentially vacuuming the debris out of the lines, until I blew/sucked the lines and got the remaining debris (assumed from old V. Modulator) out.
 

May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
Hi jpoole,

Just measured the power wire and I'm getting 12.x - 13.xV with IGN switched to on.
With engine running while in P = 13.xV, while in D(not moving)= 13.xV.
So I'm not losing power I think.

However the switching wire with IGN to on = .2xV
With engine running while in P = .5 - .7V, while in D(not moving) = .5 - .7V
On this wire when revving the most I'd get is 1.7V.
Everything changes though when driving.
It's just not being triggered while in P or not moving.
ECM is not triggering or conditions are not being met to trigger while stationary.

ECM is not arriving till Friday so I have some time to do some more tinkering.
Was thinking of just running the switching wire straight from ECM to the VSV harness just to check.
 
Joined
Sep 15, 2016
Messages
696
Location
Chattanooga, TN
That's good data on how it's behaving.

What I don't know and am curious about is what the ECM conditions are for powering up the EGR VSV circuit. I recall it being discussed but don't know if temp, speed, rpms, etc. are factored into the logic of the ECM control of the EGR VSV. If it comes to it I can probably find time to test the voltage at my EGR VSV socket while stationary later to offer comparison numbers if that's helpful.

I do know that the P0401 isn't triggered until after two drives up to speed which makes me wonder if the EGR isn't powered up when not moving? Perhaps the P0401 waits until two drive to speed cycles to ensure that exhaust temps are hot though, I really have no idea.
 

May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
I double checked and disconnected the EZ1 connector for the vsv's by the intake manifold close to the fuel pump regulator.
Measured resistance from that section to vsv harness and the other half to ecu connector and they are all about .2 -.3 ohms.

Tested for short to ground for each half and none found.
Shook the egr harness area real good from the ez1 connector to egr harness area and no change in resistance.
Wiggled ez1 as well and gave the harness terminals a slight tug to see if there are any changes in resistance and short to ground. None.

Found discrepancy in FSM for the VAF resistance on page 60 of 94LC EWD.
VCC - E2 - (2.5 -5.9 K ohm) at ecu connector
but shows on next page 61 = term 4 and 5(200-600 ohms).
When I measured resistance from VAF terminals (harness disconnected) I get 389 ohms.
When measured at ecu connector(vaf harness connected to vaf) I got the same as well.
Am I supposed to be getting 2.5 5.9K ohms at the ECU connectors?

Can anyone please verify as this has been driving me up the wall.
ECM arrived and unfortunately no change at all.

Thanks
 
Joined
Mar 22, 2017
Messages
127
Location
SoCal
If they failed you for EGR function test and only checked for vacuum in park they incorrectly failed you. The 1fze will only show vacuum at the EGR valve while driving.
 

May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
HI dugsgms... is that noted somewhere on the FSM. I've never heard or read of that. Can you point me to that?
Thank you.
 
Joined
Jul 28, 2017
Messages
580
Location
georgia
May at what rpm do they test your vehicle at?
Did the inspection tech actually hook up a guage to check the vacuum on the EGR?
Pardon my questions if they seem dumb but the states i have lived in have never had such an extensive emissions check in place.
 
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May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
I didn't see him test it with a vacuum gauge as I wasn't in view of him.
I did hear the engine die and knew he tested the EGR valve with a vacuum pump.
Well on the smog report it shows the test at idle and 2500 rpm.
He wrote on the smog report "no vacuum when revving, no vacuum in gear revving".
They only have 1 dyno track not for 4wd/awd.
Well my modulator was bad so it will fail regardless.

Double checked my ECT terminal on the ECM connector and it ranged from .5 to .8V.
Range is from .2 to 1V.
I don't have a digital temp gauge though.

Just to make sure I checked the ebay ECM by grounding the EGR terminal
on the ECU to ground and it triggered the VSV.

Can anyone please test their EGR VSV's to see if it
clicks (just touch it with your fingers) while revving a hot engine at 2500 rpm.
Don't burn yourself!
Thanks
 
Joined
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Messages
15,232
Location
OC, CA
I can’t say that I read all that, but there is no vacuum on the EGR line until the engine coolant temperature warms up to operating temp. Maybe the coolant temp sensor is bad?
 
Joined
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Messages
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SoCal
HI dugsgms... is that noted somewhere on the FSM. I've never heard or read of that. Can you point me to that?
Thank you.

Its not in the FSM, its just the way it is. Keep in mind the FSM is only mildly useful, its full of mistakes and the diagnostic routines are nowhere near as good as the ones offered by manufacturers like GM and Ford. "T" a vacuum gauge into the hose on the EGR valve, drive it, if it gets vacuum there's no point in continuing to diagnose.
 
Joined
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Messages
15,232
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In the manual for my 96, it implies that there will be vacuum on the line in park or neutral because it does not say that you have to have it in drive and all the other tests are in park.

I find the FSM very useful and accurate, especially the trouble shooting guide for emissions related problems where the potential causes are ranked in terms of their probability.
 
Joined
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Messages
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I find the FSM very useful and accurate, especially the trouble shooting guide for emissions related problems where the potential causes are ranked in terms of their probability.

Really? Let me ask you this, do you consider spaghetti-o's to be authentic Italian cuisine? Don't get me wrong, its better than nothing but as compared to others its not that great.
 
Joined
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georgia
I can’t say that I read all that, but there is no vacuum on the EGR line until the engine coolant temperature warms up to operating temp. Maybe the coolant temp sensor is bad?
this is part of the problem i had with mine whenever i would get emissions test done,i would drive around for a good 30 min and the engine temp was never high enough,eventually after revving in park at 2800 rpm the temps would come up and the tech could finish the test.
the test done one my vehicle were at idle and 2500-2800 rpm.
 

May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
Well it gets more intriguing now.

Since my leads are still connected to the ECM terminals and verified ECT
voltages on ECM were OK, decided to read live EGR terminal voltage.
To my surprise when I started the car this morning it now shows 13.6V. What!!!!!!!!!!!!
Can't be, it's not supposed to have that at the VSV on a cold engine.
We drove for a while so now engine is hot.
While idling and WOT it shows 13.6V...Nooooooooooooo
Stopped the car, had my wife start the engine, felt the vsv and didn't feel it click as I revved it.
It should have clicked!

Tracing last steps:
Replaced CPU
Key on engine off test of triggering vsv (which works).
Realtime voltage reading of ECT while driving
Replaced TPS with used TPS from ebay
Realtime voltage reading of (EGR - E2) while driving

I thought I may have forgotten to do something before installing the TPS or maybe it needs an adjustment.
But idle is fine and engine still runs good. Should I adjust the TPS?
Reinstalled my old TPS no change
Reinstalled my old ECM no change.

Then I thought I messed up the VSV while repeatedly clicking on body ground.
I removed the VSV and all tests passed again. Hmmmmm???

Had my wife start the truck and got.2V at EGR terminal.. What???
Wait....I'm holding on to the VSV.
Plugged VSV back on and 13.6V is back at ECM. How can that be?
My head is spinning at this point.

It dawned on me I did the original voltage readings WITHOUT the vsv connected and measured from the VSV harness!!!
Now I'm measuring the voltage from the ECM side instead.
Well this led me to measure voltages from both ends now WITH VSV installed.
So a voltmeter on both ends.
VSV end =0V, ECM = 13.6V(engine running on Park)
2 different readings on each end!!!

Decided to repeat individual wire measurement from VSV harness to batt post neg with VSV installed, engine running in park:
Power wire to neg battery post 13.6V
Control wire to neg batt post also gets 13.6V
Control wire to positive post -.6V

So to review my observations:
I assumed voltage at VSV end is reflective of what reading you'll get at ECM side but it's not.
At ECM(connector connected (EGR - E2)) initial voltage at startup 13.6V and changes as you drive.

Seems like everything is back the way it is.
I have relocated the VSV inside the truck and watched voltage, vacuum, and heard the vsv click.
Vacuum reading is teed between VSV and EGR valve.
My vacuum reading maxes out bet 3 to 4 in/Hg. Sorry about it getting cut off on the video.
Have not tried going uphill or downhill, all flat driving.

Can someone please verify that your vsv doesn't click(turn on) while on P with a hot engine, at least on a '94.

 
Joined
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Messages
15,232
Location
OC, CA
You can’t have two different voltages on opposite ends of a single wire if the wire is good. Measure the resistance across the wire from both ends and make sure it doesn’t change when you pull on the harness and wiggle it.
 

May

Joined
Feb 20, 2017
Messages
65
Location
Los Angeles, CA
Thanks..yes I've done that on my earlier post. VSV wires and EGR temp sensor wires are not shorted to ground and resistances were all ok. I was referring/comparing about the voltages across terminals (EGR-E2) on ecm and across the terminals on the vsv end, not each individual wire. Later I did test the voltages on the vsv end to battery posts for each wire. Just been reading about it this week and it's called Ground Side switching and confusing. There is 12v going down each wire and once the ECM triggers it via ground down the trigger/control wire, it kicks in, completes the circuit enabling the load. Saw a few videos on it and it makes sense now. I'm just a diy'er, just sharing what I've experienced.
 
Joined
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Messages
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Location
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Checking the voltage across terminals may be meaningless unless one is hot and the other is grounded. Always check voltage with one of the leads grounded.

Checking voltages on an open circuit can also fool you into thinking that the circuit is good. Check for voltage on the feed side of a device like the VSV when the VSV is energized. If there is voltage before it is switched on but no or low voltage after turning it on, then the supply wire may have a bad connection.
 

May

Joined
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Messages
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Location
Los Angeles, CA
Cool...I'll definitely try the voltage down the feed wire as it's energized as I've never tried that. Tried it only while parked, which is my issue since it's not being energized while on P. However if there is a bad connection, it should display minimal to no voltage? Just grasping concepts. Thanks.
 
Joined
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georgia
just throwing this out there,you stated pair is disabled.
is it possible a vacuum line is routed wrong,how far back are the pair lines(vac)unplugged
 

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