First off, welcome aboard. I completely understand your story. You want to keep it as close to stock as possible without getting bit by the hot rod bug. I gotcha. But, without a Scanmaster, you are proceeding blindly and are on a dangerous course that will eventually lead to a blown head gasket or 2. Even on a bone stock car with low miles it will happen. I know you say you don't want one, but you don't have to hard mount it. You could simply plug it up, put it on the dash with some velcro, go for a drive and get the data, then remove it, wiping the Velcro residue (if any) off with some Simple Green or Goo Off. Easy peasy. It's not a permanent install. Some people simply stash it away inside the glove box. You gotta know what's going on under wide open throttle or disasterous things can happen. But I highly recommend mounting it where you can see it at all times while driving. Either on top of the dash, or using one of Kirban's steering column mounts. For years I just mounted it on the dash, but now I have this mount and it's great.
With that disclaimer out of the way, here's my best ass guess as to what's going on.
Symptoms: Runs great at first, but once it gets going good, it falls on it's face, pops and stumbles, stops pulling.
- If the car still has the stock fuel filter on it, even with very low miles, it is definitely clogged and starving the engine of fuel. This would show up as a lean condition under WOT (wide open throttle) when watching a Scanmaster driving down the road and punching it. The first screen shows your O2 Mv and degrees of knock. These numbers are real time and are what you use to help diagnose a lean condition.
- The spitting, stumbling and popping could be detonation, or it could be numerous other things, such as a failing TPS sensor, failing due to age coil pack or ignition module. It could also be a failing MAF sensor, or even something as common as low battery voltage from a failing alternator or dead celled battery. All of these can be diagnosed with a Scanmaster so you don't have to start throwing parts at it. IOW, the Scanmaster will pay for itself seven fold over the long run.
- The factory vacuum hoses are no doubt dry rotted at this point. You could have one that's even crumbled and broken / fallen off. As in disconnected and causing a vacuum leak. This will lead to a severe lean condition as you loose your fuel pressure rise under WOT. This is one of the most critical vacuum lines on the engine, as without a good sealed vacuum system, the fuel pressure won't rise and the engine will go severely lean under boost. I recommend to everyone, go over all of the vacuum hoses that originate from the vacuum distribution block on the top of the throttle body. If you see any signs of dry rotted, replace every hose.
- Check the hoses on the turbo and wastegate actuator, and the ones on the wastegate solenoid the same. If disconnected or if dry rotted, R&R them asap. It can cause an over boost condition.
- Don't trust the factory dash boost gauge. It will lie. Get a good boost gauge, if you don't already have one. Stock cars with a good chip and the fuel system addressed, can typically run 14-16psi with ethanol blended 93 pump gas, sometimes even higher, but that depends on the condition of the injectors and pump. On a seriously bone stock car, keep it safe at 12-14.
- As a start, I would change the fuel filter, any auto parts store will have what you need. Then change the spark plugs. Next would be to replace the fuel pump with a Walbro 340 255lph pump and a hot wire kit. And lastly, I would run some fuel system cleaner through the system like Chevron Techron. I understand about keeping it stock and even with low miles, injectors do go bad. So, replacing the injectors with a matching chip should be high on the list for the future.
- The stock chip has got to be replaced. It was programmed with higher and more aggressive timing tables for non ethanol blended pump gas back in the "80's / "90's. Keeping the stock chip and running 93 octane with it's blended 10% ethanol can cause detonation. Don't put it back in, no matter what.
- Even though an adjustable fuel pressure regulator doesn't look stock you gotta get one. The black Accufab one, is as close to stock appearing as you will get.
You can do these things to even a mint condition car without hurting it. If this is an investment for the future, the fuel system mods are just preventive maintenance that will protect your investment. And whatever you do, don't forget to run some zinc/phosphorus additive like ZDDPlus in the oil at all times.
If you haven't read up about the spring cleaning, I highly recommend doing so.
http://www.gnttype.org/maint/basics.html
Don't mind the negative comments. These types of threads have been common for years. And I can understand and respect wanting to keep it stock. Unfortunately, it's so easy to hot rod these cars and make an extra 50 rwhp that you get the GoFastItis bug without even trying.
Get a Scanmaster, replace the fuel filter, inspect and replace those vacuum hoses, get some ZDDPlus for the oil that's in it now, put the TurboTweak chip in it are the very first things I would do. If at this point you see the car is running lean under boost, replace the fuel pump and put a hot wire kit t on it. That's what I would do if it was mine.
Hope this helps and again, welcome aboard.
-Patrick-