Need Help Understanding Code 33 (High MAP)

[jmgtp]

I’m going through the process of tuning my LX9 (obd I ecm), well Ben is tuning it I just flash the bin to the chip and datalog.

I’ve been getting 2 check engine lights lately. Code 33 (high MAP) and Code 39 (clutch switch). The 33 is what is concerning to me.

What we noticed, for example, is that at 35% throttle the kpa was at 96! If my logic is correct that means that at 35% throttle there was nearly NO manifold vacuum. This is something I’d expect at wide open throttle, not 35%, and I believe this is why the ecm is tripping the code.

The idle map reading looks normal. So, I can’t figure out why the kpa is so high at part throttle.

I’ve verified the 5v and ground signal at the MAP harness. I then brewed up my own test of the MAP sensor.

I took the map out of the manifold but it remained connected to the wire harness. Using a rubber hose I hooked up the MAP port to a hand operated vacuum pump with gauge. I then applied roughly ~20” of vacuum to the sensor. I then keyed on the motor and connected to TunerPro and I had a reading of ~12kpa. Keyed off, release the vacuum applied to the sensor, keyed back on and reconnected to Tunerpro = ~100kpa.

Here is a chart that I found online that clearly shows the relationship of KPA to Vacuum.
http://www.engineeringtoolbox.com/vacuum-converter-d_460.html
It appears my findings make sense and the map sensor is working correctly.

So, the question is, is it truly possible that I have almost NO vacuum in the intake manifold, at part throttle and lets say 2500 rpm? And if so, would this condition trigger code 33?

Where we are currently at:
The idle seems stable. Part throttle at revs under 3k rpm is very underpowered. Part throttle at revs above 3k rpm builds some torque but is still very underpowered. Using 5th gear, even at 60mph, causes the engine to shake as if it is being bogged down terribly. The last time it tripped the MAP sensor code 33, was at idle, even though the idle kpa looks normal (I believe around 38kpa).

Should I be running a compression check on the motor to be sure it is mechanically sound? But, I’m thinking it is mechanically sound because 38kpa at idle is roughly 20” of vacuum, which seems just about right for a stock cam motor. The engine also runs quiet as a mouse (the clutch is another story).

edit: forgot to mention that I also tried a spare MAP sensor and it behaved exactly the same.

[robertisaar]

sounds like you have one hell of an exhaust restriction.

that will cause nearly everything you've mentioned; low power, no vacuum with relatively low throttle, possibly the quiet motor as well....

[jmgtp]

Hmm very interesting interesting.

I'm using stock 3400 exhaust manifold into a 2.5" downpipe. The exhaust is the same setup that was on my dohc motor.
Could it be possible the cat is toasted? When I first swapped the LX9 I had a pcv issue that caused it to burn lots of oil, maybe the cat self destructed during that time?
After the cat is a moroso spiral flow, and beyond that 2 stock mufflers from a 2nd Gen Grand Prix - I don't think anything after the cat could be the problem. I can cut the cat out and weld in a test pipe for the time being. I can't think of any other way to check it. I don't have any rattling coming from the converter nor does it smell bad. it is an aftermarket ceramic mesh converter.

edit: Googled it, and it sure does sound like it can be my problem! This would honestly be a relief, I have a spare converter and I can weld. Would be a simple fix. Would running the car without an o2 sensor possibly allow me to identify the converter as the culprit? The gaping hole where the sensor would be would allow excessive backpressure to vent. I think other than an increase in noise, it would just run in open loop.

[SappySE107]

it would be nice if its a bad cat. It wants more fuel at 60kpa than it does at 90 kpa at the same RPM. Idle at 1200 cold wants more fuel and idle at 800 warm wants a LOT less fuel. So I just changed the tune to see what it does but I have to believe its a physical issue somewhere.

[Superdave]

Ben, do you want the bin and logs from my stock 3500 tune so you can compare to the one you are doing?

That MAP reading is not normal for sure...

2 questions, your 7x trigger is set to 0 degrees right? and did you extend your CPS wire or is it all stock length?

[jmgtp]

Yes, it is set at 0* but I will double check it. The CPS is a factory braided wiring, no extensions.

Really hoping for a bad cat to be the issue. My cat and downpipe are welded 1 piece but I have a ball/cone connection after the cat so it is pretty easy to remove from the car. Once I have it off it will only take 1 cut right before the cat before I can get a look at the catalyst. I think at this point it will be disappointing to see a clean mesh - I am hoping it is the problem. A bad cat makes the most sense right now.

[SappySE107]

Dave, if your stock 3500 tune looks somewhat close to a 3400 tune for tps vs map datalog values, then I should be ok once we get the physical issue(s) taken care of. I am hoping for a bad cat too (meow? hiss!).

[jmgtp]

The cat looks perfect.

The mesh is clean as can be and held up to a light I can see straight through the mesh. Looks like new. No clogs at all.

Completly stumped.

[jmgtp]

I verified the integrity of the 3 map sensor wires from the harness at the senor all the way back to the harness at the ecm. All check out with low resitance and no shorts. I plugged the ecm harness back in and with key on I verified the 5v reference signal as well as the ground, both check out good. I replaced the ecm with a spare that I have. It started up and idled just fine. I didn't datalog it nor did I drive it since the car was blocked in. I'll datalog it today. I don't think I have any exhaust restriction. The tone at the mufflers is normal and the exhaust is the same exact system that was on the car when it had the dohc motor just a few months ago. I'm using 3400 exhaust manifolds.

It crossed my mind that I could have a bent valve or something that was causing a cylinder not to seal. But the manifold vacuum at idle is normal so I don't think that can be the case. Plus the engine runs quiet, it does not make any noises at all. I'm using an airbox from a 97 lumina with a paper filter, the filter looks clean.

My only remaining hope is that the ecm was bad and the replacement ecm fixes the issue. If it does not I don't know what to look at next. Open to suggestions. Will post tonight results of new ecm.

[jmgtp]

Tried new ecm and same results. Tried old style map sensor just for the heck of it and same results.
At small throttle angles (lets say 15% TPS or less) it actually runs pretty good. It might feel a hair down on torque but it ain't too bad.

I don't know what to do. I'm trying to find an exhaust back pressure gauge just to confirm whether or not the exhaust is the problem. If it isn't a restriction in the exhaust then I have no idea what to do next.

[Superdave]

Dave, if your stock 3500 tune looks somewhat close to a 3400 tune for tps vs map datalog values, then I should be ok once we get the physical issue(s) taken care of. I am hoping for a bad cat too (meow? hiss!).

It's fairly close except for WOT, idle is nearly identical. The SA is a little different though.

OP, I'd pull the valve covers and check the rockers and valve springs...

[jmgtp]

So you think it's possible I have a loose rocker or bad spring? I guess I'm just confused by it because I have good manifold vacuum at idle and no top end noises. I'll certainly check it out though. Too bad that the placement of the icm prevents me from running the motor with the covers off. I'm not sure if the 60v6 would spray oil everywhere either. I've run my LT1 without the covers when adjusting valve lash.

I have an exhaust back pressure gauge on the way. Locally they want $150+ for it. I bought a new one in amazon for $40 and expect it in about a week. It threasds into the o2 sensor port. I figure I will drill the exhaust past the cat and weld in another o2 sensor bung so I can get a reading before and after the cat.

[pocket-rocket]

I don't think the 60v6 do splash oil around with the covers off. Otherwise you would see it spraying all over looking in an oil cap. I also primed a 3400 on my stand with a gauge on the OPSU port and IIRC, I managed something like 50 psi with my 1/2" Dewalt drill and oil oozed out of the holes in the rocker arms where the cups for the pushrods go. You could loosen the front one, have your woman start it and pull the cover off and if it sprays, put it back on and have her shut it down, just to make sure.

[jmgtp]

I got my exhaust backpressure gauge, hooked it up to the o2 sensor port and fired the car up. At idle, I register NOTHING. At 2500, NOTHING. At a steady 3500 NOTHING. If I rev it off idle with A LOT of throttle I can get the needle to spike up to about 2psi or so for a split second. So, I think it is safe to conclude I do not have a backpressure issue.

I cannot explain why the manifold vacuum goes to almost nothing at part throttle. I'm tapped out of ideas.

[robertisaar]

where exactly is your MAP sensor getting it's vacuum reading from?