View Full Version : Rebuilt Northstar, No Codes, Poor MPG

06-08-06, 02:09 AM
Background - had major oil leak straight onto exhaust and gasket leaks on both heads. Rather than just fix these things had a rebult put in.

Has TONS of power. I love the thing but I'm averaging 14.5MPG whereas before I was closer to 18 with a lead foot. I know on trips that were pure freeway I could average 23 to 26 with A/C on and cruise.

This could be a matter of just me loving the power but I don't think I'm driving much differently.

No codes, new engine internals, no codes. Slight, non consistant idle dropping a few hundered RPM every minute give or take but not like clockwork, if you catch my drift, completely staccato and sometimes idles without it. But mostly when warmed up, ever 30 to 60 seconds the idle drops from about 750 to 500, catches, and back to normal.

Throttle response is good, tapping the throttle makes her wanna take off a little harder than I remember.... but there is no hesitation when under any throttle condition hot or cold.

Could this just be a few MPG with new internals breaking in. We're literally talking less than 400 miles.

Normally I'd not ask about MPG but I want to make sure my "new" engine is working correctly. I'm sure it isn't internals - things are too smooth.. too SWEET watching that tach sweep up smoothly and quickly.


06-08-06, 07:51 AM
replace both front O2 sensors and theres 90% chance this is your problem. It sounds like it.

Also the dropping idle thing you are talking about, I remember that and it was the EGR valve. I had that all the time and always wondered. Then when it gets completely plugged it stalls teh engine every 30 minutes or so and takes 20 minutes to restart it. Clean/replace EGR valve and replace O2 sensors. I think this one is nailed down. Definately do the sensors though man, im telling you. You cant keep wasting premium like that..LOL. I knew something wasnt right before and sure enough it was them.

I got mine here. http://www.partsamerica.com

06-08-06, 10:32 AM
Ditto the O2 sensors. Coolant will damage them.

Also a possibility the catalytic is clogged.

06-08-06, 11:41 AM
Hey, guys, excellent. I was pretty comfortable with the idea of these jobs - thanks for reinforcing my thoughts. I was going throug a LOT of coolant (burning) before the engine rebuild and I'm kinda torqued that my mehanic didn't replace the O2s, but it would have added to the cost and he probably didn't wanna call me ONE MORE TIME and say, "well, this is a lot more than I thought it was gonna be!!"

I have my STS to do list now... and you're right Supreme is 3.50 here and way too expensive to be averageing 14.5 if I can get 17.5 out of it!

BTW - never had to change a cat-con. Is this a special Northstar type made from precious metals??? I've just come to expect to pay more!

This will pay for itself even sooner thanks to high gas prices... makes it far easier to swallow.

06-08-06, 12:26 PM
My brother had a Cougar that blew the head gaskets. After getting them done he had to replace both the O2's and the convertors. Coolant does not mix with either of them well. Many times they don't have to be changed but it does shorten their lives.

I'd do the sensors first.

As a matter of fact converters Are made of precious metals. Good thing they aren't made of diamonds. :bouncy:

06-08-06, 12:29 PM
I know the insides are - on the cats - just wondering if Cadillac decided to use specail golden cased cat cons!

Just ordered two 02s from http://www.partsamerica.com for $55 each. Not bad. Turns out there are only two O2s on a 1994.

06-08-06, 12:30 PM
If you have to change the at I'd check on a direct replacement. They are usually much better quality than the universal weld in replacements.

06-08-06, 02:48 PM
Check your FPR.

06-08-06, 02:56 PM
I have two direct replacements, they're Bosch I believe. If they are weld in, I'm sending them back.

I now have a 3rd thing to check.

So, I'm gonna replace the O2's, then the FPR, then the cat con. I think that is a good attack strategy.

Anyone disagree?

Thanks for all help/suggestions.

06-12-06, 07:47 AM
speaking of precous metals around here some are stealing catalytic converters off of all sorts of cars at dealerships, was story on the news.

06-12-06, 11:45 AM
Oh you got the Bosch's? I thought you were going to get the Delco's, they are what I got. Try the Bosch's anyway if you got them, as long as they are OEM Replacement you shouldnt have a problem. I just didnt think ANYTHING non-delco electronic parts wouldnt work right on these cars.

06-12-06, 12:21 PM
Oh you got the Bosch's? I thought you were going to get the Delco's, they are what I got. Try the Bosch's anyway if you got them, as long as they are OEM Replacement you shouldnt have a problem. I just didnt think ANYTHING non-delco electronic parts wouldnt work right on these cars.

I put in spark plug wires that weren't Delcos (before I knew this) and they worked perfect. They even had the exact same markings on them which made me think they supplied Delco.

I didn't see Delco's on the list of where I bought from. :rant2:

06-15-06, 07:36 PM
Replaced the front one, and it immediately cured the idle issue. Haven't driven enough to notice gas mileage yet.

The back one looks like a PITA so I'm saving it for this weekend.

silver duke
06-15-06, 10:57 PM
hi there , i was just curious how much a rebuilt motor ran you in cost. was that a rebuilt or new motor from gm? who carries good rebuilt northstars?

06-15-06, 11:19 PM
US Engine in Kent, Washington and it was $3800 with timeserts and a 7 year 70,000 mile warranty. That was before labor....

silver duke
06-16-06, 02:25 PM
thats a good price and good warranty also. thanks for the info