1999 Boxster Starts and then Dies...real head scratcher

Ok so Im really hoping someone on here has had a similar issue and can guide me because I feel like Im out of options. I have a 1999 Boxster with tiptronic. Car has 108K and engine has around 57K due to it being replaced after the IMS went out. Got a CEL for the MAF reading low. No worries I'll just replace it with a Bosch. After putting it in, the car starts up and runs for 20-30 secs and the dies. Here is my lengthy list of attempts to find the issue: 1. Unplug the new MAF and it runs fine. 2. Put in the old MAF that has failed and it still runs fine. 3. Changed the CrankPS. No difference 4. Changed the O2 sensors. No difference 5. Changed the AOS. No difference 6. Coil packs and plugs were already changed. 7. Cleaned the throttle body butterfly. 8. Changed the Idle Control Valve ICV. No difference 9. Checked everywhere for vacuum leaks. Now here is the weird part. I removed the ICV, and placed a piece of tape covering about 80% of the intake hole. Plugged in the new MAF and the car runs fine. This is telling me that the new MAF and O2 sensors are either giving bad information to the DME , or the DME is possibly bad and misinterpreting the signals from the MAF and O2 sensors, shutting down the IDV. The signals on the upstream O2 sensors are reacting as expected on the scope, after they warm up. So should I start looking to repair/replace the DME ?? I see a couple of places on the web that repair them. Thoughts??

It is possible you got a bad replacement MAF. Just because it is new, doesn't mean it is good. Do you have a buddy nearby you can swap with as an experiment? Maybe try to get a different one under warranty?

I ordered a new Bosch MAF from Amazon to test that theory, knowing I could return it. Same issue.

Based on my own experiences, it wouldn't hurt to connect the new MAF disconnect / re connect the battery and see if that has any affect, it's free and easy too.

Thanks for the reply. I did that as well. Tried disconnecting the battery and the fuse to the DME based on other posts. I let it sit for an hour before reconnecting. Still no change.

Have you read the codes?

Yes. The only code being thrown is the MAF code...because its not plugged in.

I’m thinking here… are you getting spark? Alternator die’n? Battery going with it?

Wouldn't the spark be compromised with the old MAF plugged in too though? As well as the rest of that list too.

I’m picking my brain a little…

The fact that it runs with the MAF disconnected means it's running OK on a pre-set map in the ECU chip. It's when active mixture control is brought into play that it screws up. The running OK with no MAF eliminates vacuum leaks since it would run poorly on a pre-set mixture map if the system had a vacuum leak. Also eliminates plugs/coils/AOS, leaving us with the components that provide mixture information to the ECU when it's in active control mode. Those would be the O2 sensors, the primary ones. Could there be a bad O2 sensor? Dunno - what tool is being used to read the ECU? I'd like one where I could watch the O2 sensor activity, and see if they get into switching mode (meaning the system is reading them and responding). You need a tool that will graph output vs time. If the O2 sensors don't get into switching mode (and I suspect with the MAF connected they won't), I'd be looking at a defective O2 sensor OR an exhaust leak between the head and the O2 sensor. If one sensor does work correctly and the other one doesn't - you've now isolated it to one bank. I know it's a lot of work - but you just put in new plugs? Ideally removing the plugs and "reading" them should give a clue if it's a problem with a single cylinder or multiple cylinders. If you do take them out - number them as you remove them, then take high-resolution close-ups of the working bit (tip and electrodes).

I was leading to plugs on my previous post, then I erased it… plugs will tell you if it’s spark or fuel.. generally

Don made an excellent well thought out pursuit list. I too was thinking it's an O2 sensor, I'd suspect one even more if any are @ 108K miles. The reason I didn't type that is an 02 sensor should 100% throw a code. If she was mine I'd want to see a scan with another tool. Maybe the switching mode Don spoke of means the 02 sensor won't throw a code but could still be an issue so that's a key takeaway. Here's the sunny news, it runs like it should mechanically, that would make me feel good about the car and paying a professional if diagnosis continues to evade you.

Could be through’n an O2 code because of the mixture being either rich or lean depending on spark or lack there of or the MAF not functioning… maybe that’s the point your getting at tho

The only code is "missing MAF" when he runs it unplugged. I'm saying the 02 might have a failure not recognized, it is an OG Boxster , it's codes may be limited in contrast to newer versions.

Got it…

Wow great responses...thank you!! Concerning the O2 sensors I replaced them with Bosch and waited for them to heat up before graphing them. Both pre-cat sensors are receiving and responding now as expected. I did graph them before I replaced them, and even after warm up they were both flatlined...which is what lead to the replacement. Coil packs and plugs are new as well. The car runs great with the MAF unplugged. No hesitation in performance. The wife loves the car so I may have to take it in for a full diagnostic as suggested previously. The part that still eludes me is the removal of the ICV from the mix, and taping over 80% of the intake hole on the throttle body. MAF connected, it runs fine in this scenario by eliminating the ICV.

What I still haven't seen (did I miss it?) is what tool is being used to read the codes? Let's see if I understand the statement "I removed the ICV, and placed a piece of tape covering about 80% of the intake hole. " (and the car ran fine with the new MAF).. what's this mean? Does this mean you electrically disconnected it and then blocked the air inlet to the ICV with the tape? That's my WAG.. Let us think again. No MAF puts the ECU into a pre-mapped mixture mode. It probably also sets the ICV into a preset non-active position. When working correctly the ICV only responds to idle speed and it will open and close to maintain the correct speed. If the pre-mapped MAF failure mode map also sets the ICV into a "preset" position - there's a big clue here. You need a tool that looks at the output of the ECU to the ICV. Based on this - my WAG is - when in closed-loop operation (MAF functioning, O2 sensors functioning, IVC functioning) the ECU is getting confused as to idle speed. Depending on the era of the injection system it can pickup engine speed from one of two sources - camshaft sensors (unlikely to have them since it doesn't have variable valve timing) or the crankshaft sensor. When you remove the MAF from the system the ECU defaults back to a map that's designed to get you home, and that map likely uses default settings for mixture, timing and ICV opening. It doesn't necessarily mean the MAF is bad, removing it just triggers the preset map and it ignores anything that might cause it not to run. I don't see the crankshaft sensor on your list of things that have been checked or replaced. And what diagnostics tool are you using?

Thanks for that lengthy and informative reply. You actually might be on to something with the CPS. I replaced that back in March. Checking the receipt I "thought" I had bought a Bosch replacement. Turns out I didn't. IF there is one thing I have learned with the car, its dont use cheap replacement parts. The sensor was over $140 so I thought it was a Bosch. To answer your question on the scanner, I have an inexpensive ~$150 handheld that does some graphing on the major items like O2 sensors. It is by no means a fancy, expensive diagnostics tool. I'm going to try and get a hold of a Bosch CPS and try that before going to the next step of having someone run a full diagnostics. I'll take this CPS back and apply the $$$ to a Bosch and see what happens. I'll keep you posted.