Now that I have my car on the road I've been digging deeper into smaller issues that plague it. Now I'm fixating on MPGs and what seems to me like a rich mixture.

Relevant info: 1999 D1 SD 4.0/auto, 165k miles
- Idle is ~860 (too high it seems), it doesn't idle BAD, but it's got some funk to it.
- Consistently throws P1447 Evap Purge valve code. I ordered a new one, but would like to find a test procedure in case that doesn't fix the issue. No other codes.
- No thermostat. I'm putting in a 180* unit and doing a coolant flush once it arrives. Engine runs 175-190 on a normal day. Lately it's run cooler due to lower ambient temps
- The right bank manifold to y-pipe gasket is leaking somewhat. I'm fixing this, but my research says it's probably not causing too many issues.
- When I fill the tank all the way gas leaks from somewhere in the area of the front of the tank. (PO mentioned Chevy fuel filter mod?)
- 2" Terrafirma lift, 265/75R16 MT tires
- Quick napkin math says I'm getting ~13 MPGs, which to be fair is probably about where it should be

Looking at my UltraGauge my short term fuel trims are in the roughly -15 realm with some fluctuation the whole time the truck is in closed loop. My upstream O2 sensors are cycling like they should. The downstream bank 1 sensor reads 1.020v all the time, and the downstream bank 2 sensor usually hovers from .4-.6 (this seems perfect). Long term fuel trims are both steady 0s. I'm building a smoke machine to test for vacuum leaks later this week.

So obviously the ECU is trying to pull fuel on the short term. Further confirming my rich mixture theory is the tailpipe is black, and there's even a small ring of soot on the outside of the pipe.

Is there anywhere else I should be looking on top of fixing the obvious issues? Is it best to let sleeping dogs lie? What other questions should I be asking? It runs good, no power loss that I'm aware of, no super obvious fuel smells while running, I'm just a bit of a perfectionist and don't like leaving "performance" on the table.

Sorry for spamming the board, and thank you in advance for any advice.

 

Its running rich due to the truck temps fluctuating so much. It will still run rich on occasion with the 180° unit.
You may be able to reset the adaptive values and fix the idle, its possible the IACV needs to be pulled and cleaned or replaced.

Gas tank has probably been hacked on, should be easy enough to pull the panel out of the rear cargo area and check the top of it.
13mpg is pretty solid.

Gasket isn't a big deal, but I would go ahead and take care of it.

 

I just drove about 2,000 miles cross country in my 96' automatic - averaged 16.7MPG - the car is all stock and had lots of maint. so I would like to think it is typical albeit I do have some rare instances of the VSS failing - happened a couple of brief times on the trip - not sure if that had any impact ? tires are stock size and pretty close tread-pattern to the original Michelins - there were some head winds and some tail winds so I assume that evened out - no mountain driving - temps were in the 40's-60's for the most part - I usually filled with 91 or 89 octane - think most had 10% eth

 

re-flash the ECU. There is a thread on it around here somewhere. Effectively you will delete all the adaptive values the ECU has learned (some bad habits perhaps) and start from factory fresh.

https://play.google.com/store/apps/d...ms.gemsutility

 

I've experienced cold coolant temps causing rich mixtures before, and it's the main reason I'm installing a thermostat, but I'd expect to see the fuel trims going positive if that's what the ECU were actually trying to compensate for.

A fillup with 89 octane made things look a hair better. I'll try resetting the ECU once I get some repairs done.

Thanks!

 

Just following up here. I put in a new y-pipe gasket, did a coolant flush with a new 180* thermostat and hoses, and reset the adaptive values.

The idle has cleaned itself up a lot, and the fuel trims seem to have come down on the short term, but I'm still not getting very good mileage (definitely not 13)

The fuel leak, and probably the P1447 code, is the fuel pump o-ring seal. I have one on the way, we'll see what that does.

Overall it seems better, and I feel better knowing it has new hoses, and a functioning thermostat (the old one was stuck open).

I'm finally running out of things that need to be addressed. It could use a camel cut, and the terrafirma kit leans to the right something awful, but it does the job I bought it to do exceptionally well!

 

i think the Terafirma kit has different components for left and right sides to compensate for the driver, maybe they are backwards??

Your exhaust leak can cause problems with the fuel as the converters will get too much o2 , and also could contribute to the temp issues.

Must be something with these 99 D1 as I have been fighting with many of the same problems. believe it or not, my 'list' is getting shorter too!!
I still get a cyl 8 miss code when I first start out, I reset it and it is fine until the next day... Coil and wires look brand new... i can reset it without even looking at the tool now...

 

I knew the terrafirma kits were side dependent going in, and still had to swap the springs on the front.

Unless they're talking about a RHD vehicle and I still have them backwards? This is going to bother me now...

 

sorry....