Hello Lads,

I recently bought a 2002 ZR2 with 4.3 engine 162K on it from the original owner in his 60"s that really took good care of the car, I have a pile of repair receipts. He put in a crate engine 30K ago and a rebuilt transmission 20K ago. When I bought it owner reported getting 17 MPG combined. The car runs great starts right up idle is at 625 RPM or so, No pops, backfires, step hard on the gas and it is gone!

Since I got it I was getting 12 MPG in city driving, I put in a new air filter, not an oil bath type, replaced a broken vacuum line that went from intake manifold to a T that goes to the charcoal canister I also put in a new PCV valve, Now I am getting like 8 or 9 MPG! No check engine light, and a hand held scanner says there are no codes present or stored from engine.

I am going mad and broke filling the tank. I can find no gas leaks, no smell of gas in engine compartment, and only a slight rich smell in the exhaust. There is no EGR valve on this model. It had new plugs, wires and distributor cap and rotor at the engine swap. The fuel filter looks almost new, with no mud or stains on it.

I went to two shops and a Dealer and explained the situation, I got two "it beats me" and we could do a $450.00 tune up but can't promise anything since it isn't throwing codes. I am thinking of replacing the Mass air valve to see if that has a difference other than that I am stumped,

If anyone out there can help, brother please speak up.

Thanks,

DB

 

As far as PCM is concerned, the system is running correctly and is controlling the fuel mix or the SES light would be on (SES light does work with KOEO, right?).

Big differences in fuel mileage are often due to dragging brakes (stuck caliper pistons) or plugged catalytic converters (but converters usually turn on the SES light). Calipers on these are almost one-use then replace, because of the aluminum body - aluminum corrodes from nastys in the fluid and then piston sticks when piston is pushed back and new pads are put on. I would look to those first before throwing parts at the problem. Ease of flow through exhaust can be checked by monitoring engine vacuum at off-idle vs maybe 3000 in park. If vacuum drops significantly at higher rpm, the exhaust is restricted. Brakes you have to jack it up and turn the wheels/inspect things to see if they are dragging.

One other thing I have heard of happening is that the snorkel on the front of the air filter box (where it goes through the radiator support) can get bent over and this causes a restricted air intake - this could also cause a big reduction in fuel mileage.

 

 

Sorry, I don't have that information at this time, all I have ia a Autel handheld reader.
SER light is indeed functioning, as it lights up on KOEO and goes out at start.

 

George, Sorry, I meant to say I haven't read them yet. It is 102 outside right now so I am going to wait awhile to go out and read them then I will post.
Thanks

 

George,
I think I have these correct,
At idle of 625
Short term ft 1% 0 to 1.6
Long term ft 1% -7.08

short term 2% 0 to 1.6
Long term 2% -6.3

3000 RPM
short term FT 1% 0 to 1.6
Long Term 1% -10.2

Short term 2% 0
Long term 2% -5.5

I hope this makes sense to you, as honestly it does not to me

Thanks,
Donald

 

Negative values mean that the PCM is subtracting fuel from the factory injector pulse calibration in order to correct the fuel mixture, but in your case not so much that it turns on the SES light. I know that George likes these, so I'll let him take it from here!! He will get you fixed up in no time! Interesting that you have 0-1.6 for short term fuel trim, 3 times at 2 different RPMs on two different banks.

 

George,
If you are out there, can you sign in with some suggestions?
Thanks,
DB

 

As Les says, your fuel trims are reasonable so this Will not not directly give us your solution but it does rule some things out and helps us know where to go next.

What normal fuel trims means is that your engine is able to manage the air fuel delivery to create proper emissions levels and that the level of fuel delivery to achieve that is pretty close to theoretical values - given the levels being reported by the sensors. So one of the explanations is a bad sensor(s) where the prime suspects would be oxygen sensors in the exhaust, the engine coolant sensor or the MAF sensor. There is a test process for each of these but let’s actually save that for later and look at some of the other causes that are just as likely and less involved.

Did you follow up on Les’ suggestion of dragging brakes? If not take it out of gear at 5 mph and coast to a dead stop. Does it stick at the end or go to 0 mph very slowly at the end.

Does the truck have high quality tires in good shape with no odd wear patterns?

How is the power level of the truck in general?

Do you ever smell gas? Is there any evidence of a leak from the fuel tank up to the engine or in the intake manifold at rest?

Have you performed a compression test?

Is the air cleaner new?

What do the spark plugs look like?

Have you performed the fuel pressure test in particular looking for a leaking regulator?

Are there ODB codes or pending codes?

Once you have rules these things out, we can move on to testing sensors.

 

George,
here is the odd things, when I fist bought it I was getting 12 MPG, three weeks ago I put in a new air filter, a new PCV valve and repaired a cracked vacuum line that went from intake manifold to a T fitting that goes to the charcoal canister. Then Mileage dropped to 8 MPG. There is no smell of gas anywhere, but just a slight rich smell in exhaust no visible leaks. No dragging brakes, as I went down a street here in town at 20 MPH and shifted to neutral and coasted nearly the whole block till it rolls to a stop. The engine starts immediately excellent idle at around 630 and acceleration with no hesitation, pops or odd noise, you stand on the gas and she screams goodbye, and watch the tranny shift to the tachometer at correct speeds. No smoke in exhaust The tires are Les Schwab Wild Country with lots of tread left and correct pressure. Had the front tires checked and balanced on Monday at LS and they saw nothing in the brakes, only found my idler arm going bad.
There are no codes, no pending codes, and no stored codes. Check engine light is working, SES lights up at key on and winks out at start up.. I don't have a way to check fuel pressure or compression myself, so it will need to go to a shop for that. Haven't pulled any spark plugs yet, as the heat is crazy at 102 degrees at Noon. No smell of gas in oil.
I don't know if it matters to this engine, but if I pull the power plug on the MAV sensor it will stumble for a millisecond and keep running. The dash temperature gauge never seems to go above 200. I was under the Blazer with a super bright from tak tointake line and not a single drip or wet spot, So I am stuck as to intake or air sensors

Like I said, I do not believe the guy I bought it from would knowingly screw me, as this is a guy that took care of the slightest problem right away, There is a new crate motor and transmission just 30K ago and big bucks in new Rancho shocks all around, new ball joints and CV joints just 10K ago. I have all the repair tickets in a file that came with the rig. Any suggestion?

Thanks,
DB