WBO2 to diagnose stumbling issue at low revs and high load?

[monkeyodeath]

I'm at wit's end with an issue I've been dealing with on my '86 3.2 for over a year.

Basically, when the engine is warm, I'm having a major stumble issue and occasional hard start.

The stumbling manifests itself at times of low RPM and high load -- like, for example, shifting into 2nd after taking off from a stop in 1st. The car really stumbles badly as I try to pull away in 2nd.

I've replaced the intake manifold gaskets, swapped to a known good AFM, had the DME looked over, tried different ignition coils, swapped the FPR, replaced the fuel pump, and had the injectors cleaned.

In some ways, the situation has improved -- for a while, the car wouldn't even start or idle and now it does that fine, but the hesitation at low revs/high load remains, and there's a very occasional hard start issue. Passed smog last week with pretty fantastic numbers for whatever that's worth -- so at least the engine is generally running OK.

I'm wondering, at this point -- does it make sense to use a wideband O2 sensor to help diagnose this problem? To at least see what the mixture is doing when the car stumbles? Kind of feeling like I'm running out of stuff to replace here and any help/advice would be super-appreciated!

[PeteKz]

I'm not a Motronic guru, but...

Stumbles are usually caused by a lean condition. If you put in a wideband sensor, that's almost certainly what you will find. The question is why? You may have significant vacuum/intake leaks, so make sure your hoses and connections are all good. The usual way to test for intake air leaks is by spraying starting fluid, or CARB cleaner, or propane around the intake manifold and hoses; or a smoke tester. Your car has power brakes, which use vacuum from the intake for power. check those hoses and connections.

However, the oxygen sensor should be operating in closed loop under the conditions you describe, so it should adjust the mixture if it's too lean, unless: (1) the leak is big enough that it can't correct for the additional air, or (2) the O2 sensor is faulty.

After verifying that you don't have vacuum leaks, then think about replacing the O2 sensor. They do wear out. How old/how many miles on it? You could have "pretty fantastic numbers" on the smog check becuase it is running lean due to a faulty O2 sensor.

[nospiners]

A similar stumble in my car was solved by changing the fuel filter!! (Prob not the problem here)
The frustrating issue with the Motronic for me , is the inability to use a scanner for diagnosing problems. AEM has a wideband o2 with a controller for OBD II SYSTEMS , so I wonder if anyone has ever thought of trying to come up with a solution . Very time consuming ,step by step diagnosis required. Ultimately ,only experience , through lessons hard learned , gets you there .

[monkeyodeath]

Good call on the intake leak, I suppose it makes more sense to look into that first, as well as the O2, which is the one part I have yet to replace.

I think I'll take the $$ I would have spent on the WBO2 sensor and start by getting a smoke test done (or, more likely, a better smoke tester than the DIY jobs I've been using).

I'll try to reproduce the issue and then quickly smoke test the intake to try to find the leak.