I'm at a loss and hope that someone can point me in the right direction. This is my mothers car and I'm doing everything I can to keep her from taking this to a shop as this can be quite costly...

The problem:
First off the car has to reach operating temperature for the problem to present itself. Once driven (if it doesn't stall) around the block and parked, attempting to re-start after 10 or so minutes the car fails to stay running. It turns over every time - but when running the rpm's are erratic, the engine stutters, hunts then flat out stalls. Resetting the ECU or simply allowing the car to cool down (couple of hours) allows me to get the engine started again.

The code:
Scanning the car returns a code 33, "MAP/MAF Sensor Too High". No other codes are stored.

What I did 1.5 years ago (preventive maintenance):
Plugs, plug wires, coil packs all replaced with OEM parts. The IAC, TPS and EGR, MAP, coolant temp sensor and O2 sensor. New plenum,throttle body and rocker arm gaskets were installed as well as a new PCV. I had a local exhaust shop install a new catalytic convertor (I don't have welding equipment). I also had a local transmission shop rebuild the transmission as it was leaking fluid quite heavily.

What I've done (past week):
I've replaced the MAP sensor which didn't solve the issue, I can only assume that the lack of proper vacuum (4.66mV) immediately proceeding the stall is attributed to the code being thrown. Normal values @ idle appear to be 1.66mV (+- 3 or 4mV). I visually inspected all the vacuum lines exiting the throttle body. I've replaced the crank positioning sensor - what a pain, be careful not to break it in 1/2 during removal (mine cracked). I've verified that the fuel pressure (at the rail) during prime (2 seconds) is approximately 40psi and 37 when the car is running correctly. When the car attempts to stall the fuel pressure never drops below 30psi and you can see that the car is adding fuel.

I thought that the problem was resolved when I replaced the crank sensor (car ran smoother) but when I drove it on Sunday it stalled on me driving up a hill. I have a service manual and I've read the diagnosis section closely but without replacing sensors that were recently replaced last year I'm running out of options here. When the car doesn't stall it runs smooth as glass when driven and at idle.

Sorry for the long explanation, any help is greatly appreciated.

 

My next approach will be to diagnose the EVAP system. I remember removing the gas cap the other day after a test drive and observing that an abnormal amount of vapor discharge occured. I'm not sure if this was negative or positive pressure - regardless it was enough to warrant inspection.

I'll be checking the Vapor Canister Purge Valve(solenoid) and the Vapor Canister/Filter.

Here is an excerpt from the GM Service Manual:
The venting of vapors to the canister is controlled by an in-tank pressure control valve. The fuel vapors vent from the fuel tank to the tank tube of the canister. THE CANISTER IS LOCATED IN THE LEFT REAR WHEEL OF THE CAR BEHIND AN ACCESS COVER. The canister purge is controlled by a remotely mounted normally opened pulse width modulated solenoid. (The Part is referred to as a Purge Solenoid)


The ECM operates a normally opened pulse width modulated solenoid valve which controls vacuum to the purge valve in the charcoal canister. Under cold engine idle conditions, the solenoid is turned on by the ECM which closes the solenoid and blocks vacuum to the canister purge valve. The ECM turns off the solenoid valve and allows purge when:


Engine is warm – After the engine has been running a specified time – above a specified road speed – above a specified throttle opening


The in-tank pressure control valve a combination roll-over, integral pressure and vacuum relief valve, is located with the fuel sending unit in the fuel tank. When vapor pressure in the tank exceeds 1” Hg (5Kpa) the valve opens and allows vapors to vent to the canister and then be purged. When the tank pressure drops below the opening point of the valve it will close, keeping vapors in the fuel tank. The valve provides vacuum relief to protect against vacuum build up in the fuel tank and roll-over protection to prevent liquid fuel from entering the canister during normal driving maneuvers.


Results of incorrect operation ---- Poor idle, stalling and poor drivability can be caused by, Damaged Canister, Hoses split, cracked and or not connected to the proper tubes


Evidence of fuel loss or fuel vapor odor can be caused by ---- Liquid fuel leaking from front fuel lines, cracked or damaged canister, Disconnected, misrouted, kinked, deteriorated or damaged hoses, air cleaner or air-cleaner gasket improperly seated.


Visual Check of Canister--- Cracked or damaged, replace canister – check filter at bottom of canister. If dirty, plugged or damaged, replace filter

 

I'm a t a loss on my son's 89 sepreme too. I'll try checking the vapro canister.

Our codes are the same as yours (33). yet I wonder why the codes are given, since the MAP and MAP are only sensors and not necessarily indicative of the actual cause.

 

Yes, I visually checked the EVAP canister and the respective hoses and they all seem (again visually) fine - the hoses were not cracked and everything appeared intact. I've decided, since the vehicle is as old as it is, to replace all the injectors. This should be a quick process and I hope to start & finish it this coming Saturday or Sunday. I'll report back regardless.

If the injectors don't remedy the problem then I'll probably entertain the idea of swapping out the EVAP Solenoid.

Keep in touch and let me know if you stumble upon a solution.

Thanks

 

Before I forget, in response to your statement about the MAP sensor. I can only theorize that code 33 is false due to the fact that when the car begins to lose power - the vacuum in the system drops to such a point that the MAP sensor reads a high voltage (low vacuum source) thus throwing a code 33.

 

We dismantled the MAP and there were no obstructions to be seen. All the gaskets were intact too. Put it all back together and reset the ECM. No codes after a quick start up.

Took it for a test drive. All was well as usual for the first ten miles. Then it acted up again.

Limped it home and checked for trouble codes - still no codes.

However before we left to test drive it, I released the pressure in the tank. After the test drive there was pressure in the tank after only about 20 minutes of driving.

It doesn't make sense. It runs fine, then acts like It only gets enough gas to idle, but doesn't get enough to allow it to rev. It routinely runs fine for 20 minutes before the problem occurs.

 

Yes, this is definitely frustrating. As much as I hate throwing parts at this problem, I'm forced to do exactly that. The injectors are on their way from RockAuto along with new gaskets for the intake plenum/throttle body and EGR - I'll definitely report back whether or not that resolved the issue (I'm hoping it does but I feel it won't). I keep leaning towards lack of fuel delivery as I can't believe that a sensor(s) alone would cause the vehicle to stall - engineers are aware that sensors are prone to failure due to time and environmental elements, so I expect that the PCM would execute a safe routine if the sensor itself is providing erroneous results (e.g. exceeding thresholds or non-responsive).

 

Yes, a bad sensor/missing sensor input can cause a stall, I know of this happening with MAF sensors specifically. While engineers know about sensor failure, accountants might not!

 

Well, today was going very well - removed everything including the fuel rails in about an hour. Went to mount the new injectors to the rails and discovered that one of them had been dropped on concrete (head of injector was bent), sealed within a package and placed within a box. I sent pics to RockAuto today so hopefully they'll get me a replacement quickly.

4 of the 6 old injectors looked as expected, the other two had a lot of oil and grime residue that caked around the outer edges of the spray tip.

Contemplating replacing the fuel pressure regulator since I have the rails removed. I'll report back when I re-assemble everything.

Andy

 

Update:

Received the replacement injector from Rock Auto yesterday when I got home from work. On close inspection, all the vacuum lines looked to be in great condition as well as the PVC (just wanted to make sure there was no obstruction), I installed the new injectors and verified there we no fuel leaks prior to re-installing everything else. No leaks during prime, at that point I re-installed everything and replaced all the gaskets using a kit from Fel-Pro. Torqued everything to spec (plenum bolts, EGR bolts, throttle body bolts etc..) and verified all the sensors were plugged in.

Started right up, took it for a drive and couldn't get it to stall. No idle hunting - nothing, ran very smooth. However, after about an hour of driving around the neighborhood (stop and go) I noticed a slight electrical smell coming through the A/C vents. I looked down at the voltage meter and had about 2-3 bars. Took the car back to my house and realized that the alternator must be on its way out - it had a strong electrical smell right at the alternator and was too hot to even touch not to mention the supercharger sound caused by the bearings I assume.

So I'll be ordering a replacement alternator from Rock Auto and replacing that this week.

I hope this is the end of the issues I was chasing down, only time will tell - perhaps today wasn't the day it would stall but tomorrow might be another story. I'll keep this thread going with periodic updates, if anyone has specific questions please let me know.

I saved some money by purchasing re-manufactured injectors which saved a lot of money; GB REMANUFACTURING @ $38 per injector (includes $10 core charge per injector) as opposed to $102.79 for an AC Delco.

Andy