I am working on my friend's 99 4.3 4wd. I checked the SES light with a scanner and it came up with three codes in this order. p0300 random misfire, then p0327 knock sensor low voltage and finally p0410 secondary air malfunction. It seems to start/idle ok but missfires badly when trying to accelerate/pullhills. It sounds like it is knocking but after checking the real time data on my scanner, the ECM is not pulling timing. I swapped out a known working MAF and no difference. I did a fuel pressure leak down and observed a small drop (~ .5 psi) over fifteen minutes and no change the next fifteen. I was convinced that it was a fuel leak in the plenum but the leak down pretty much ruled that out.

 

I'd start by addressing the P0327 code. It is possible that it is not pulling timing when it should be. The PCM will advance the timing until it sees some predetonation (knock), then back it off. If it is seeing no knock, then it will keep on advancing the timing, quite possibly causing the problems you are experiencing.

P0410 is typically a problem with the air injection reactor (AIR) pump which should be located under the radiator near the passenger side frame rail (atleast that's where it is on a 4wd model). There is a technical service bulletin that states that the design of the intake tube may be insufficient at keeping liquid from reaching the pump. Water in the pump will result in corrosion and will result in a locked up pump if left unattended. Some people have had luck removing the AIR pump, disassembling it, and giving it a thorough cleaning inside. I would bench test it to make sure that it powers up properly before reinstalling it.

 

I just got off the phone with the owner and he told me that his gas mileage has gotten a lot worse. Which brings me back to the fuel leaking scenario.I checked the fuel pressure when the motor was cold over a 30 minute period. Should it be at operating temp? How much of a drop pressure over what time frame is a problem?

While checking connections around the dist cap, I found a connector not plugged into anything. It was lying on top of the dist cap. It has maybe a six inch wrapped wire and the plug has only one pin. I traced the wire back to a large loom that comes from the passenger side valve cover area. Tomorrow I will swap out a known good coil and see what happens.

 

A single wire connector sure sounds like what is supposed to go to the knock sensor to me...

Here is the connector view:

*click for full view*

 

ding ding ding ding ding

Know if you could make my hand small enough to get behind the dist cap.

 

Well, get that thing hooked back up and see if your friend gets better mileage and/or performance from it. I suspect that the disconnected knock sensor has been causing much of the performance/mileage concern. The AIR pump won't affect mileage.

Your testing of the fuel system leak down showed very good results. I do not think there is anything wrong in the fuel system.

 

I went to hook up the knock sensor and I can't find the sensor. Turns out it does not have a knock sensor. My friend's dad swapped motors in this blazer. Supposedly, the new motor is a 2000 and he had problems with the wrong knock sensor ordered for it. He installed it without and it was running ok for almost a year prior to this. Only problem has been a never ending SES light. So much for that being the fix.

I swapped a known working coil and module. It seemed to run better but still missfires. I pulled the dist cap and although worn, I didn't see any problems there. My next move is to pull the plugs, inspect and replace. Then do the same for wires. I think I have narrowed this thing down(crossing fingers).

 

Replaced wires/plugs and that fixed the missfires. The air pump was a little different. Turns out the check valve was stuck open on the passenger side. This caused hot exhaust to go the wrong way through the pipe and melted just about everything in it's path. This created a pretty nasty exhaust leak that further complicated the diagnosis. It now runs great and the owner is happy.
Thanks for your help, Kyle and everyone else who has posted their problems in the past. It saved me a lot of guess work. Everything that I purchased was needed and I didn't have to "throw" parts at it.

 

Very good! Your bill is in the mail! LOL