GS300 Won't Start with MAF plugged in - no codes
 

Recently had the head rebuilt on my 2001 GS300 with the stock GE motor. I acquired this car in ron-running condition and removed the head myself and had it re-done by a professional (6 bent valves from the engine running out of time as a result of an accident). At first, the engine wouldn't start after reassembly and the only code indicated the throttle control motor on the throttle body. I took the motor off and spun it to confirm that it was shot. I got a lightly used OEM throttle body from eBay and put it on, and it worked just fine and after clearing the code with my Blue Driver the code never came back. However, during troubleshooting, I could only get the car to start and run by unplugging the mass air flow sensor. I cleaned it with MAF sensor cleaner and still no luck with it. The engine sounds fine when running, not like it's out of time or anything like that. Even drove it around the block. I got a brand new sensor from O'Reillys today just to try out and had the same results - didn't start. While driving with the MAF unplugged, the car reported codes P0100 (Mass or Volume Air Flow Sensor "A" circuit), P0110 (Intake Air Temperature Sensor 1 Circuit Bank 1), and P0101 (Mass or Volume Air Flow Sensor "A' Circuit Range/Performance). I plugged the new MAF in and cleared the codes and the engine would never start even after disconnecting the battery for a while. Ran the "Enhanced Diagnostic" from the Blue Driver with the new MAF in and the engine reported no codes despite not wanting to start.

Is it possible that something simple like a blown fuse is causing something not to report correctly? This seems a little bizarre to me, but I am not that experienced with Lexus/Toyota and this is my first project car. I checked all the harnesses near the intake and everything seemed to be snug. I'm not sure where to look from here and would like to avoid throwing money at unnecessary parts until I trust the car a little more. It may be worth noting that a few parts are missing, one being the top plastic slit that is removed to service the filter, and the little scoop that tucks into the grille (I'm not sure if these things may be having a dramatic enough effect to prevent the engine from starting, the air filter could probably be replaced).

Oh, and one more thing. Does the Mass Air Flow sensor have a gasket between it and the intake? Mine wobbles around a bit. Don't know if a leak there could cause this as well.

The MAF should definitely be secured down with a seal. Air will leak past and cause a code, but I really don't think that would cause a no start, just poor running after it was running. Some things to look at are the igniter over on the drivers side. It's not uncommon for that connector to be broken and making a bad contact. And if you suspect an injector, a cylinder power balance test would be the way to determine if an injector is at fault. Once running, don't forget about a vacuum gauge. Look for indicators of valve issues etc.

I do suspect an injector but also not sure if that wasn't just because of a lack of a MAF period. While driving without the MAF the engine hesitates around 3000 RPM. I will check out the igniter. Need as many suggestions as possible, I have to drive an hour to where the car is from where I live so it's a big PITA.

Honestly might try to limp it home so I can be in my own shop

is it possible the intake pipe that holds the maf sensor is backwards? or the maf sensor is installed backwards? you did not specify what kind of intake you have. factory, or cold air intake. fyi, the maf sensor has to be installed correctly to calculate the airflow across it properly.

It is the bullet-shaped MAF on the stock intake. I assume the rounded end goes forward?

check under the intake resonator (the box that sits right above the headers). I found it to be a VERY common area to get brittle from the heat of the manifold and either crack or completely fall apart. I can almost bet thats what your issue is. Theres a metal shield under there, when you remove it, you might find something. Also check the nipples of the 2 hoses going into the intake. Those too get brittle and crack at the base.

Finally got the car home, just said to hell with it and ate the tow truck cost. Sprung a coolant leak trying to drive it home. At least now I can do some better troubleshooting and reporting.

So for the last three days I’ve been trying to diagnose on why the car will not run when the mass airflow sensor is plugged in. I finally had a friend come over and he got in the car to crank it up while I try to run a few tests under the hood because it would not stay running had to give it constant gas. As he was revving it up I was hearing sounds of air like a vacuum leak. I grabbed the old trusty can of brake cleaner and spray the engine. The gasket that mounts to the intake and the throttlebody was sucking too much air I took the throttlebody off and applied some anaerobic sealer and put the throttlebody back on and the issue is no longer. Hope this helps for any future technician/mechanic.

The cap on top of the air filter has to be installed correctly. Even a small gap 1/8 inch will cause air leak and pulls a P0171 code.

With the missing cap I suspect too much air gets in the intake, the MAF produces a value that confuses the ECU, therefore it won't start.

Tape down the opening over the air filter to see if that improves. The little horn-like plastic part in front of the filter may not have that much effect, but engineers would nto put it there if they don't really need it to do something..