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Thread: Question about post-cat O2 sensor Reply to Thread
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Topic Review (Newest First)
04-02-2013 10:40 AM
VillageT I went through a similar issue with a Ztec. It turned out it was the hose to the PCV. I had also sprayed all the hoses but this one fails underneath were you can't see it and you can't spray in that area. We finally took it to a shop with "smoke test" equipment and they found it in a couple minutes. This equipment puts smoke into the engine and it is easy to see where the leaks are.
04-02-2013 10:25 AM
sailor Joboo - thanks for posting the "resolution" of your old thread!

As you mention, this can help others when trying to work through similar issues.
04-01-2013 09:19 PM
amc49 Luck with that..........

Just went through similar myself, tries to die mostly and erratic around the idle area, turned out to be the TPS switch in my case, now idles perfectly. I had no codes showing whatever.
04-01-2013 12:04 PM
Joboo For the benefit of anyone trying to work through a stall issue, my solution to this problem ended up not having anything to do with O2 sensors or vacuum. Thanks to a "pending" code in the ECU (which I can't remember now) I started looking at the ignition. The wiring harness the carries the wires that run to the coil run adjacent to one of the exhaust re-circulation pipes (I think that's what it is - it has a braided insulation wrap) and it had melted through the harness and exposed some of the wire. I spliced in a new piece ... I thought that hadn't done anything (idle sounded off), so I gave up and finally took it to a shop. They drove it 30 miles and told me everything was running fine (fuel trims, etc), and I've been driving with no issues. The idle still doesn't sound as stable as I would expect, but it's running fine and - most importantly - no stalls.
07-24-2012 11:00 AM
Joboo
Quote:
Originally Posted by BC_ZX3 View Post
Since the removal of any vacuum line will admit air into the intake that has not been accounted for by the MAF sensor, you should produce a condition lean enough to cause a stall or low idle.

You should be plugging the vacant intake port, not the line, initially. If you plug the port and the idle improves, examine the fitting, hard plastic line and any other solenoids or valves in the path for leaks.

Another thought: Also check your EGR valve is closing and seating properly. If you apply vacuum to the fitting at the top and release you should hear an audible clunk. Not a definitive test, but at least tells you the spring loaded internal plate is moving. You could also try blocking the EGR intake port entirely.
I took the EGR off and bench tested it. Seemed OK - plunger moved freely and it held a vacuum.

I will try the vacuum test. I just assumed the symptoms of the various systems not having vacuum would be worse than my existing problem, but I guess that's not the case.

Thanks.
07-23-2012 10:46 PM
BC_ZX3
Quote:
Originally Posted by Joboo View Post
Besides codes, should I notice any change in performance when removing and plugging vacuum lines?
Since the removal of any vacuum line will admit air into the intake that has not been accounted for by the MAF sensor, you should produce a condition lean enough to cause a stall or low idle.

You should be plugging the vacant intake port, not the line, initially. If you plug the port and the idle improves, examine the fitting, hard plastic line and any other solenoids or valves in the path for leaks.

Another thought: Also check your EGR valve is closing and seating properly. If you apply vacuum to the fitting at the top and release you should hear an audible clunk. Not a definitive test, but at least tells you the spring loaded internal plate is moving. You could also try blocking the EGR intake port entirely.
07-22-2012 11:29 PM
Joboo
Quote:
Originally Posted by amc49 View Post
A plugged cat will run worse at higher rpm if it is bad enough to affect idle.

Common to check for vacuum leaks by plugging all fittings you can temporarily and then testing idle quality. Once the plugins are isolated as good then you go to gaskets and/or possible cracking somewhere to leak unmetered air.

See if the front O2 sensor is stuck low below .5 volt at warmed up idle. Indicative of a vacuum leak.
That's a good tip. I'm pretty sure the front O2 sensor is getting above .5V .. in all cases it seems to be between .1V - .9V, but I will double-check.

Quote:
Originally Posted by BC_ZX3 View Post
Yes.


While the vacuum tubing is some kind of special hard plastic - crimps fairly easily btw - the connectors clustered at the top of the manifold are simple push on rubber fittings (except for the big EVAP fitting). You can pull them off one at a time and inspect. Try blocking the port temporarily and see if the idle improves to try isolate any offending line.

Did you replace the PCV elbow and valve (front of the engine) when you repaired the collapsed rear hose?
I did replace the valve itself, but not the front elbow. It seemed in OK shape - no obvious splits or collapses.

I did another spray test this afternoon and noticed no change in idle. I also fiddled with all the connections, and again, nothing.

Besides codes, should I notice any change in performance when removing and plugging vacuum lines? I want to make sure I'm diagnosing the original problem and not noticing artifacts of the missing vacuum...

Thanks again for all the help.
07-22-2012 10:43 PM
amc49 A plugged cat will run worse at higher rpm if it is bad enough to affect idle.

Common to check for vacuum leaks by plugging all fittings you can temporarily and then testing idle quality. Once the plugins are isolated as good then you go to gaskets and/or possible cracking somewhere to leak unmetered air.

See if the front O2 sensor is stuck low below .5 volt at warmed up idle. Indicative of a vacuum leak.
07-22-2012 10:41 PM
amc49 A plugged cat will run worse at higher rpm if it is bad enough to affect idle.

Common to check for vacuum leaks by plugging all fittings you can temporarily and then testing idle quality. Once the plugins are isolated as good then you go to gaskets and/or possible cracking somewhere to leak unmetered air.
07-22-2012 09:05 PM
BC_ZX3
Quote:
Originally Posted by Joboo View Post
Is that the one that goes into the sensor on the left side of the fuel rail?
Yes.
Quote:
Originally Posted by Joboo View Post
Everything else looks like reinforced lines with special connectors.
While the vacuum tubing is some kind of special hard plastic - crimps fairly easily btw - the connectors clustered at the top of the manifold are simple push on rubber fittings (except for the big EVAP fitting). You can pull them off one at a time and inspect. Try blocking the port temporarily and see if the idle improves to try isolate any offending line.

Did you replace the PCV elbow and valve (front of the engine) when you repaired the collapsed rear hose?
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