Since I finished the suspension, I started getting a stumble at idle- nothing horrible but noticeable, then it was verified with P0300.

Anyway, I知 seeing these on the fuel trims. They seem all over the friggin map and the long term seems way high too. But then again, I don稚 really know what I知 reading and what it means. Can someone edumacate me?





















Those seem way all over the map and don稚 seem like my last Disco. I知 assuming I need coils for the P0300, but this has me a bit perplexed. Any ideas.

 

I'm not expert re: fuel trim, but short-term fuel trims should fluctuate between positive and negative values as the ECU adjusts fueling in response to O2 sensor voltages. The values displayed on the UltraGauge are expressed as percentages above and below zero. It's my understanding that fluctuations between -5% and +5% are "normal".

My understanding of long-term fuel trim is more limited, but the fact that LTFT on both banks is >20 is notable. That indicates the ECU is dumping fuel into the cylinders in excess of 20% of a "normal" level. That's concerning, and in many cases indicates a vacuum leak in the intake after the mass airflow sensor (MAF). This can cause the multiple misfires indicated by the P0300 code.

If I were you I would do a smoke test on the intake to look for a leak. That can be done with a cigar and a length of tubing connected to the port on the intake adjacent the brake servo vacuum connection and the PCV connection. I did this on a friend's truck several years ago. If your truck has SAI you'll need to disconnect a line at that port; otherwise it should be capped.

I also notice that in some of the pics of your UltraGauge the circle in the upper right corner is closed and in other cases it is open. The circle indicates whether or not closed-loop fueling is working. The open loop indicator should appear only for the first few minutes after a cold start or when the truck is decelerating.

And finally, I don't understand why you are assuming you need to replace the coil packs. Don't just throw parts at it. Diagnose, diagnose, diagnose.

 

You are a hero and gives me the place to start. And I know better on the diagnosis- thanks for the reminder. Some quick research said the coils could be it, but you couldn稚 be more correct.

Some more info:
- after these pics were taken I replaced the MAF and air filter and the values were similar but not as radical

- this happened after I did the suspension in the front. This is of note as it happened after I buttoned it up and went for a drive. And why that is of note is because I didn稚 remove the hose going to the idle control valve and I wondered if by folding it over I goofed it I may have introduced a vacuum leak.

Thus, you are a rock star for correctly pointing me where I need to be. I値l do some checking there and report back.

 

FYI. My mechanic says LTFT should normally be no more than 5%, otherwise a vacuum leak is indicated, just like min01 said. Could also be caused by low fuel pressure or other injector low flow conditions.

Maximum value for LTFT is 25%, so the leak is substantial. Both banks being equal is a clue about where the problem is.

 

Damn, you guys are geniuses. So I started doing some investigating on fuel trims and you guys are filling in the blanks.

A couple points I learned:
- Positive numbers mean how much more fuel needs to be added in order to get the correct fuel mixture
- Negative means it is leaning out

Those are really over simplified and I’m doing more reading, but when coupled to the data you guys have presented already, the UG pegged at 25 on a short drive tonight for both banks on short term and it is staying over 20 for long term. As you guys have said, this denotes a pretty healthy vacuum leak and the fuel system is trying to compensate.

So then, I did a quick visual on the hoses going to the idle control valve and they look good. And nothing really changed in the other boot from the MAF and it looked OK. Where else should I be looking? Lastly, although I know of the smoke test, what it does and how everything reacts and all that, I’ve never done one. Any advice on the particulars?

Thanks again so much for guiding me. This forum continues to be one of the best sources for D2 masochism. Although I have to admit, this is most likely something I did and not a design flaw or anything like that.

 

I do this sometimes when I’m on a research binge and do multiple posts, so bear with me.

I found this video:


(Ironically when I thought it was the coil packs, I pinged Gerry to see if he had any relocation kits left)

So that gets me to the particulars of the smoke test. Now I just need to find where I might have a leak. I mean with the values of the fuel trim, I gotta think it’s going to be pretty glaring. Again, any advice on some of the “regular” places to look would be great.

The only thing that gets me is that I thought vacuum leaks usually caused the idle to climb, and mine just had a mild stutter at idle and ran perfect through the rev range. However, it’s starting to seem like it as the revs climb. Where it is especially noticeable is when in reverse and just backing up slowly.

 

My last vacuum leak was when I forgot to reconnect the tube from the fuel pressure regulator. Fuel trims came right down after it was connected. I also had a bad seal between the upper and lower intake plenums, once upon a time.

In your case, I would cap off any tube that is not essential (e.g. vapor canister, EGR) and probably use a hand pump with a gauge to make sure the power brake is not leaking. Process of elimination.... Exhaust system leaks before the O2 sensors can also mess with fuel trims, and low fuel pressure. Start with the easy stuff.

PS. I saw a Scotty Kilmer video where he used a cigar to do a smoke test....

 

Holy cow. Gerry's method is not incorrect, but in my experience it is waaaaay more complicated than needed with all the fabricated equipment.

One thing his vid did remind me to mention is that you do want to disconnect the intake tube just after the MAF and seal it off. You'll be blowing smoke into the intake from the port I pictured and if the intake tube is not sealed off the smoke will move upstream and past the MAF into the airbox. I don't recall what I used, but I do recall seeing people just put a rubber glove over the open end of the tube. I suppose some plastic wrap sealed with a rubber band would work fine as well.

Once that's done, simply take a big puff on the cigar and blow smoke into the intake via some tubing connected to the port in the pic I attached. Then, look to see if and where the smoke seeps out of the system.

Places (eight places, actually) where there may be vac leaks are the fuel injector O-rings. If that's the problem you should see smoke coming out from under the upper intake. But hopefully you'll find a leak in some of the tubing or more likely at a connection.

 

I agree completely with everything you said. I’ve already got the little pump and the other “parts” shouldn’t be more than $10, so I’m going to give that a go.

I’m pretty convinced I caused this when I folded over the hoses in doing the driver’s side shock last weekend, so that’s why I want to try this. As I drove it for another quick stint the other day, the short terms trim values seemed a bit more correct after it warmed a bit, but the stutter was still there at idle.

Something else I’m going to try is cleaning the IAC valve too if I end up pulling the hoses off.

The SAI seems to be strange too at times as there are times when it is has been sitting that the pump doesn’t fire, and there are other times when the pump starts to run well after at operating temp and I come to a stop at a light for a few seconds. I suspect I need to clean that thing out and replace the filter as I’ve seen in some threads, but I think that is a different issue.

As said, diagnose diagnose diagnose, and that SAI deal was going on before this so I don’t think related. Again, I’m going to stick with a vacuum leak since this was an acute issue just after the suspension as I’m a firm believer things don’t just fail. I’ll be heading out for some stuff and will post some pics of what I put together and come up with a little later today hopefully.

 

OK, Gerry is a genius! I was able to replicate his contraption and use it. WOW! Damn thing worked! The best part? Zero dollar fix as I just screwed up (or not as it were) the main intake boot.

See the video:

https://longtallsallygs.smugmug.com/...ry/i-vzvWzgJ/A

I think it’s utterly hilarious. To confirm everything, I reset the adaptations via the Nanocom and then read the trim values on the UG. Started out as cold pegged at 25, and then as it warmed up it went down to normal numbers and cycled from small positive values to negative on the short trim and I probably didn’t run it long enough to get long term values.

Here are some pics of the smoke tester:









I already sent a note to Jerry thanking him. He thought it was hilarious too.