Poor MPG on a'95 Buick

[jetice]

My '95 Park Ave is getting really poor fuel economy, from what other people say it should get. Right now it is averaging 14 city, 19 hwy. The background on the car is we bought it from a young kid with the rods hanging out the oil pan. We found a replacement out of a '02 Grand Prix with 55,XXX miles, and had it installed at a professional shop. The car had a few check engine lights, bad O2 sensor and bad MAF. We replaced the MAF and that one cleared up, but the O2 was a pain. We replaced the sensor, all the wiring to the sensor and the pcm, and the code STILL persisted. We finally put a variable resistor in-line to clear that code. We also just had the Trans rebuilt by a local shop because the OD wasn't working. Everybody we know and have talked to is stumped as to why it is getting such poor fuel economy. It runs fine, has good power, and there is no indication that anything is wrong with it. The only thing it does do is at an idle, if you put your hand on something that is solidly connected to the car, it shutters very slightly for about a second, and then idles smoothly for another second. It just sits there and repeats.

Does anybody have any ideas? I would really appreciate some help.

[TrickyTransAm]

Well first and foremost, did you replace the upper intake manifold with an aftermarket Dorman, and lower intake with GM grey gaskets? If not, I'd start there just to be sure you don't have an intake leak (Which..if it had Dexcool through it, it's leaking)
If they re-used the 95 nylon manifold - I wouldn't be driving it right now - at all. Get the Dorman.
Ive got a pictorial guide I created if you need it.

Did you replace the PCM?
What about obvious tune up stuff, air filter, PCV, Delco O2's. Any brake calipers hanging up creating drag? Tires pumped up properly?
What about the Cat?
Usually what can happen is the bearings wipe out from the coolant entering the crankcase and that's how you throw the rod in these engines generally... but what most don't replace is the Cat - as it's having to digest all that coolant thats in the cylinder and it boogers up the Cat.

What's with the shutter part - You mean if you put your hand on the engine it shutters, then idles smoothly?
If it's any help, my 98 Bonnie with the 04 Grand prix engine didn't do much better than 25mpg. They're tanks.

[jetice]

I will have to check on the intake, and even if they did re-use the intake, it would be the '02 intake.

It does have a brand new (and flashed) PCM
New Plugs, wires, filter, couple new coils, MAF, O2 sensors, Fuel filter (few more things I know I’m forgetting)
I will also have to check on the cat
And the shuttering, if you put your hand on the dash or arm rest at idle, the whole car shutters very lightly (like if you had a tire 1/2 ounce off at 50 mph) for a moment, then goes away for a moment, then comes back, and it just repeats.

Edit: Just read about the Bonnie, i think they had a different gear set then the buicks.

[TrickyTransAm]

None of the GM intakes were good. The early ones were horrible, the later ones were better, but still required replacement due to the EGR tube making the water port separating wall very thin and brittle. You either get the misting action going on which slowly destroys the engine or it pops and your radiator ends up in your crank case.
The other thing I learned after repairing many of the 3800 series II engines, is you have to use the GM thicker "Grey" gaskets P/N 89017816. Fel Pro gaskets tend to leak after a while. I haven't had the GM gaskets come back, and they run around $60.
The Dorman Intake Part Number is 615-180 for a 95-04 Park Ave 3800 Series II, runs about $89 online.

You could spray brake cleaner or carb cleaner around the intake and see if it revvs up.
Did you clean out the IAC in the throttle body? It might be crudded up causing the surging.
Just some thoughts.

[jetice]

Is the Dorman PN the same for the Grand Prix?

[TrickyTransAm]

Yep.
The Lower intake stayed the same, thats why the upper has a year range of 95-03.
The only changes (that I'm aware of) were the EGR stovepipe in 2000 was reduced in diameter to try and help with all of the melt out issues.
In 2004 they started switching things up. 2004 Prix got the electronic throttle with aluminum upper intake, but the block/heads stayed the same. Bonnie's got picky on engine configurations, 05 up got hairy with Series III

So if you want the ultimate 3800, get the 04 Grand Prix motor, as it had the forged rods, larger ratio rockers, slightly bigger cam, and bolts right into any 95-04 car.

[jetice]

So i should check:
The IAC(do you have a diagram of the location of this?)
The Cat
The intake(spray Carb cleaner around the separation?)

[TrickyTransAm]

IAC is on the throttle body, looks like a round canister, black with a 4-5 pin electrical connector, 2 TORX bits. Spray with a little PB Break away before loosening.
Don't push down or pull on the plunger. Just check for heavy carbon deposits. Carefully clean them off with an old toothbrush and brake cleaner or sensor safe solvent.
Check the IAC seat in the throttle body for any carbon chunks, debris etc. Clean that out, then re-seat the IAC.

The CAT needs to be checked by an exhaust shop by a pressure test... but if it runs fine, you might be ok yet.
Where the Nylon meets the aluminum and the lower manifold meets the head - do this when the engine is COLD NOT HOT.
No need to burn the car down by having flammable liquid sprayed on top. Keep a fire extinguisher handy just in case.

[jetice]

Ok, thanks

[MP81]

Yep - exactly what he said.

I have a friend with a '95 Olds 88, and the first term at school, we hydro locked it after dynoing it then proceeding to drag race it against my car twice on Bluff street.

Long story short, replaced lower intake gaskets with the new style - hydro locked it again, and found out the upper intake caused the problem. Replaced that with the new design (complete with new EGR stove pipe) and he says it's never run better!

It's not even a hard swap to do, unless your engine is filled with coolant.

[WMCC Gary]

19mpg is horrible, I avg 27mpg out of my 1996 Olds Ninety-Eight with the 3800

[jetice]

19mpg is horrible, I avg 27mpg out of my 1996 Olds Ninety-Eight with the 3800

Yea, it sucks! The is Cruise set at 75mph on I-69, then i get to school and park it. Then drive back home the next weekend. It is virtually no city on it. I have even Hyper-mileage it a couple times, with no difference.

[jetice]

Just check with my dad, he said it does have the Dorman Upper intake, and it does not leak. He said it is not advancing the timing far enough.

[1BADAIR]

which 02 did you put the resister in. if its in a front(not after cat) then it has no clue what its doing
what code was it?
did you use the original injectors or the new engine stuff. pretty sure they are not the same.

[jetice]

1: Have to check, dad wired it
2: I ll have to check, i don't know. now that i think about it, we had a bad knock code that would not go away, maybe that is what we put the potentiometer on. At the beginning, we had like 5* of retard at idle b/c of that.
3: I'll have to check on this too

Edit: I will see if I can make my dad an account and get him on here to tell you guys the exact details.