midiwall

Hi all;

I have a re-occuring check-engine light on a '96 Limited.

After each mod I've done (Helix PowerTower, deckplate and now the silencer mod (thank you Dr. Z!), I've reset the ECU by pulling the EFI fuse. I've always waited for it to reset (either the "click" if I'm still under the hood, or the 15 minute delay), verified that it cleared the light, and noted the rough idle.

After the ECU's figured it out (e.g., smoothed idle), the check engine light has always come back on. The lamp had been out up to the first mod I did (the Helix), and seems to now seems to prefer being on.

Any ideas? Dr. Z, I think you have the same set of mods under your hood, is your check lamp on all the time?

The truck is running fine ("great" even!), and I don't think that anything else is going on.


Thanks all,

Mark

Dr. Zhivago

First off, I'm glad you like the ISR Mod. Secondly, I do NOT, nor have I to this day, had the MIL trip on my truck. I would suggest taking your truck to Autozone, if there is one near you and having them put the analyzer on it to see which code or codes are being produced. Then post back here with your code and we'll go from there.

The light will NOT go off until all codes are cleared from the ECU. Have them clear the codes while you are there. If a code was tripped before and the codes weren't cleared, and even if the problem no longer exists, you'll still have the MIL glaring at you.

Hope this helps.
Dr. Z

midiwall

Oh Yeah.


Sigh... well, I guess it's good that I said something then.

The closest AutoZone to me is 15 miles North of here, sounds like a good reason for a drive.


Okay, so I take it that pulling the fuse doesn't necessarily clear the codes? i.e., the ECU has FLASH RAM in it to store things across power cycles?

I'll get up there tomorrow and have them run and clear the codes. Anything that's "real" I figure will show up again on the trip back.


Thanks for the reply, and for the help.

Mark

Dr. Zhivago

You are correct in assuming that it has Flash RAM. It stores codes until cleared thru the OBDII Diagnostic Connection. And yes, it is a good reason for the drive.

Plus, as you mentioned, anything that is still a problem will show up during the trip back.

Talk to you then. Good Luck!
Dr. Z

midiwall

And the answer is...

The store I went to is brand new, and he didn't have all of his tools yet, but he did have enough to read the "P" codes. I won't pretend to know what else he could have read though.

He also said that he wouldn't erase them, only read 'em. Anyway, this is what he read:

300, 301, 302, 303, 304, 306
171

He looked up the codes for me as well (way cool of him!) so it seems that I'm misfiring on cylinders 1, 2, 3, 4, & 6 (from the 300's). And, I need an oxygen sensor (the 171), but I don't know which one (pre or post Cat).

I offered up that the misfires could be a result of my plug and wire change about 4 months ago. He suggested that I pull them all and check the gapping, make sure that the sensor is firmly seated on the plug top, and that I didn't trash the wire order. I'm a bit confused though, I'd figure that the truck would be running REALLY rough if I had screwed up 5 out of 6 cylinders (and it's actually running quite well).


Thanks again Dr. Z.

Mark

Dr. Zhivago

Hey Mark,

Well, since the DTC's (Diagnostic Trouble Codes) that are in the ECU weren't erased, you don't know if there's still a problem with your truck. Since it's running great, it's possible that the codes were generated at some point when you did a tune up or changed your plugs or something.

I would now advise you to call your local dealer to ask them if they can perform a reading of your DTC's and clear them for you. It's possible they would do that free of charge.

You need to get them cleared to see if they will rear their ugly heads again.

Hope this helps.
Dr. Z

Jay

midiwall

I think all words help me given my being a novice at this.

Thanks Sir.. I'll give he locals a call and see if I can sweet talk them.


Ya' know, if it helps... When I pull the EFI fuse, the ECU does reset in the sense that the check engine light goes out. But, after running for about a minute, it'll come back on. Is that a result of it reacting to the codes that weren''t erased or is it actually finding that something is still funky?

I think you may have answered that in pieces above, but I wanted to pose the direct question to be sure I was putting your previous info together correctly.

Thank you,

Mark

Dr. Zhivago

It is just reacting to the codes that are stored in the ECU. Pulling the fuse does not reset them. They have to be cleared by accessing the OBDII interface. The light takes a few moments to come back on more than likely because the ECU has to come out of "Open Loop" and it takes about 45 seconds to a minute for that to happen on a cold engine. I'd bet it has a series of self tests it performs during "Boot Up" as well.

And, you're very welcome.

Dr. Z

Jay

I mistakenly informed DrZ that cose P0171 was O2 sensor. Im sorry.

Ive change my mind on this topic.

Ill assume you can clear the codes by pulling the EFI fuse. (I thought you could).

DTC code chart

300-306 Misfires
171 System Lean

So basically you system is too lean, causing a lean misfire in all cylinders (with a lucky cyl #5).

Ill bet you created a vacuum leak (171) when you installed the helix power tower. Which is causing the random and specific cylinder misfires (300-306).

midiwall

I get credit for that miss as well. The guy at AutoZone told me O2 sensor, but in my wanderings last night, I found the DTC charts and now see that's it's System Lean (as you said below).

I'm REALLY glad I didn't spring for the $141 for an O2 sensor set at AutoZone.


I like this idea - it makes sense and lets the pieces fit together fairly well.

It's pouring rain today, and I have no inside spot to work on the truck, so pulling the tower will have to wait for tomorrow (fingers crossed!).

Thanks Jay!


As a side point, I did a bunch of net searching last night and came up with some interesting finds in the way of OBD II readers. I've talked to Dr. Z about them offline, and he suggested that I post up what I found for all.

I'll do that next in a new thread and keep this one open for my own personal fun.

Dr. Zhivago

Speaking of the vacuum leak thing, your HPT kit came with 2 Throttle Body Gaskets did it not? Mine did. One for each side of the HPT. I also made sure to tighten the new bolts that came with the kit securely. Hopefully you only generated the codes at one point and that things are back to normal. Good luck with your Troubleshooting.

Dr. Z

midiwall

Hey all!

I wanted to post a follow-up/close to this thread...

I got my OBD II reader on Friday (see this thread) and cleared out the codes. They came back while I was sitting there idling, so I cleared them again, and they stayed gone - for a while.

Today (Sunday) I was working on my air horns (they're in, and they work, and I'm thrilled!) and had to pull the battery to get physical access to a mounting place. So, that cleared the ECU. When I started the truck back up, it was of course doing the rough-idle-while-I-figure-it-out thing and the MIL light came back on. I haven't pulled the reader back out to see what they are, but I suspect it's the misfiring.

I've lost the light for today (and it's cold!) so I'll work on it more this week, but I figure that I'll start with checking the plug gapping and wires, and then look at the install of the Power Tower (thanks Jay!) and see what's up.


Mark

midiwall

I know this thread is pretty stale, but I wanted to close it out... again.

Jay, you were right...

Last night, Dr. Z was looking at a pic that I had taken to support a description of a new mod, and he noticed a nipple that didn't have a hose on it. I checked it this morning, and sure enough, the nipple was sucking air (versus the hose that should have been attached to it). I found the hose, attached it, cleared the MIL and it looks like life is grand. My rough idle cleared up, the idle "cozied" in, a loud whistling is gone (I always thought it was the Helix), and life seems to be a lot better.

My Chilton's doesn't have a clear shot of which hose it was, but the nipple comes off of the bottom of the throttle body at a right angle, and is pointed striaght back towards the rear of the engine. The hose runs from there to about the middle of the back edge of the manifold.

My guess is that I pulled the line to get the Helix on, and just never hooked it back up. That's only been... lesse... 18 months?


So... Thank you Jay. I'm pretty green at this, but learning.

Jay

Dr. Z told me today. We had a grand time patting ourselves on the back.

Glad ya got it fixed. And best of all it was free.

FattyCBR

Mark-

In case you're wondering that hose that was disconnected sounds like the idle air control hose. I had the same problem as you, except when mine got disconnected my truck wouldn't even idle.

midiwall

And rightly you should! Beer's on me if I ever make it down your way.


I was wondering, and this probably explains why I've always deemed my "rough idle after ECU reset" to being normal. sigh.

Thanks guys!