355 Struggling with Intermittent 1211 codes and failed smog

Hi Everyone,

I've been trying to fix this issue for several weeks now and every time I think I've found the problem, it turns out I've fixed something that wasn't the problem. Hoping some of you smart folks can point me in the right direction.

1995 F355 2.7

Car runs great. No apparent misfires or other running issues.

Passed CA smog 15 mph test easily but badly failed 25 mph test. High HC and CO indicating a rich condition. Car throws intermittent 1211 code (Lambda Regulation) at idle and very low load only on both banks. Not always together but in the same types of low load/ide conditions.

Though the car failed rich, what I found was that the o2 sensor signals (both of them) are indicating a lean condition at low load/idle. Because the car is a 2.7 I don't have the ability to look at live data. So I put a probe into the black sensor wire of each o2 and watched the voltage. It cycles normally between 0 and 1 volt at medium and high throttle openings. At low throttle openings and at hot idle the voltage drops to around .12 volt and triggers the CEL. I ran the wires into the car so I could watch while driving and the result is the same.

What I've done:

- Fuel Injection Corp rebuilt the MAFs
- New o2 sensors
- New plugs (the old ones looked ok and were uniform)
- Smoke tested intake, found leaks at Plenum Gaskets on each side (the last mechanic did a bad installation job and some of the gaskets were deformed. Fixed those (unpleasant job)
- Fixed broken vacuum line at one FPR
- Smoke tested exhaust. No leaks at headers (Tubi) or anywhere near the o2 sensors. There was a leak at one side of the muffler which I don't think matters.
- Unplugged the SAI pump on the longshot chance it was pushing air at the wrong time.
- Swapped ECUs with my other car

Given this is a global problem affecting both banks I'm scratching my head at this point. Since the car behaves normally at higher load, I'm reluctant to think I have a fuel problem (injectors, pumps, FPRs, etc). I would think those would make things worse as load increases and car behaves normally in those conditions. Unfortunately, my fuel rails don't have schrader valves so I can't check fuel pressure.

What am i missing????

 

No, I haven't had the injectors cleaned. My sense is that if they we clogged, I'd see issues worsen as load increased. But maybe I've got this wrong? My car is doing the opposite - signal issues at low load, normal signals to ecu at anything above say 1/10 throttle openings.

BTW - I forgot to add new coolant sensors for each bank to the list of things I replaced.

 

If they were leaky, why would the o2 sensors be sending a too lean signal to the ECUs?

 

The o2 sensors are indicating a lean mixture at small throttle openings/ low load. There's little fluctuation in the voltage signal to the ecu. It hovers between approximately .12 and .14 volts. When the throttle is open more and there's more load, it cycles between near 0 and 1 volts as you would expect.

That said, the car runs rich enough to fail the 25 mph test which low load. It almost like there extra air in the exhaust fooling the o2 sensor to tell the ecu to richen the mixture. But the smoke tests don't indicate any leaks which would cause that. I smoke tested both intake and exhaust

 

Thanks Tim. It's only been about 2000 miles since the last smog which it passed easily. I did a compression test around that time which looked excellent.

The search continues.....

 

Here are the smog results.
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Thanks Tim. That's a great point. It has a Capristo bypass valve for which the default position is open. If it's not closing, that could cause an issue for both banks. I'll check that today.

Last nite I actually isolated the vacuum system from the line that feeds the vacuum pump including the SAI from the engine. No dice. Everything is normal there including the pump operation.

 

I have 2 sets of cats. Tried them both - same result. I did take a temp reading - both sides around 400 degrees when hot. I don't think they're overheating which makes sense as the issue only pops up at very low load/low throttle and who drives a 355 that way.......

Multiple Italian tune-ups. No difference unfortunately, but I did at least enjoy them.

 

A huge thank you to everyone! Tim and Michael called it. And yes, I got extra grey hair out of this adventure.

Capristo bypass valve is not getting vacuum. I put a mitivac on it at 8 inHg and took the car for a drive. No more cels and the voltage signals from the o2 sensors fluctuate normally now at all throttle openings including idle. Cats now at around 575 degrees. Throttle response improved.

Now to determine if the vac solenoid is bad.

Thank you again all!

 

That's helpful, thank you. I've got vacuum at the vacuum tank so it's looking like the solenoid.

I'm assuming I'll hear a click with connecting to the 9v battery.

 

After all the endless hours of troubleshooting over the last several weeks, it ended up being a poor electrical connection at the solenoid. Working fine now. I wish I would have checked there first. I know I will if there's a next time