Thread: Sudden Drop in Power and MPG

I finally have resolved this problem. Turns out it wasn't one thing, but three.
First I tried everything that made sense. That didn't work. I tried the few suggestions I got and that didn't help. Strangely enough, there were days that the problem seemed to have cleared up. Then it would go right back. The fuel filter was changed and while it didn't make any difference, it was full of junk. The first item I discovered was a perished O ring in the fuel quick connect at the injector rail.



This is the only one of the 3 that can be linked to changing the plugs. I noticed it when the engine was cold and I had the hood up. I smelled gas and when I looked, there was a puddle on the intake manifold. I turned the key on and it was pumping out. I replaced the O ring and that at least was corrected. It didn't fix the main issue of poor throttle response. That ended up being a combination of 2 other issues. The IAC and the TPS. I cannot believe the TPS wasn't the first thing I checked. Probably because it all happened when I replaced the plugs/wires (which I actually replaced again—I now have a spare set). I also did a compression test and found an average of 184.5 psi, with a high of 190 and a low of 180. Not too bad for an engine with 215,000 miles on it. Anyway, the IAC was replaced when I did the L36 to L26 UIM conversion, but it was a Delphi part. This time I got ACDelco. I've been reading a lot of reviews that state the Delphi parts only last a short while before problems reappear.



I'm not sure if the problem (heavy build up on the valve) is a result of the TPS being bad, or if it is yet another issue, but there definitely was a problem with the idle. That's all back to normal now.

To recap, I replaced the pugs/wires, the coils, the MAF sensor, the fuel pressure regulator, the O2 sensors, the downpipe, the fuel filter and the IAC. I will replace the IAT and ECT sensors when they arrive this weekend. I haven't run a full tank of gas yet but I expect it to improve also. What an ordeal.

I created a spreadsheet with the sensors and the symptoms to narrow down the possibilities. I'll add a link for it as soon as I find a home for it online.