16-18MPG - i'm stumped/ noises + overdrive hiccups - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-03-2008, 11:17 PM   #1
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Default 16-18MPG - i'm stumped/ noises + overdrive hiccups

I have had 16-18MPG on this car (1992 SSEi) since I bought it about 2 years ago. I take my MPG at every fill up so these numbers are accurate. I live in Northeast Ohio. It is winter now, but MPG was about 17 during the summer. This is mostly city driving (about 90-95%). Over the past 2 years I have done a lot to this car. Here is a list of everything with dates.

1. Heater Core (Summer 2006)
2. SC drive unit (Fall 2006)
3. 190 Thermostat (Fall 2006)
4. Replaced all hoses (2006)
5. Gas tank (12-28-06)
6. Gas straps (12-28-06)
7. Cam sensor (4-23-07)
8. O2 Sensor AC Delco (4-26-07)
9. Fixed hood + trunk light (4-26-07)
10. PCV Valve and O-ring AC Delco (5-9-07)
11. Starter (11-1-07)
12. Oil pressure sensor Junker (11-1-07)
13. Replace LIM, SC, and TB gaskets. (Winter 2007)
14. Cleaned TB, SC, and LIM in parts cleaner. (Winter 2007)
15. Cleaned MAF Sensor (Winter 2007)
16. Front right wheel bearing (1-11-08 )
17. Monroe Struts, mount, bearings, and boots. Both front (1-11-08 )
18. Wix Fuel Filter 128,000 miles (1-11-08 )
19. Turn signal flasher (1-11-08 )
20. ELC Air Compressor relay (1-13-08 )
21. NGK TR55 Spark Plugs (1-21-08 )
22. Autolite Pro Spark Plug Wires (1-21-08 )
23. Removed both rusted support rods from exhaust pipe. Welded a rusted/broken bracket back to the exhaust pipe. (1-21-08 )
24. CAI. Rx-4750 K&N Filter. Polished aluminum pipe. Silicone 3.25 to 3 coupler. IAT drilled into filter. Spliced IAT harness for it to reach. (January 2008 )
25. Driver door lock actuator - Junker (Summer 2007)

-Oil and oil filter is changed regularly.
-No current leaks.
-MPG is mainly on the low side near 16. Driving like a grandma and even with some highway driving and it STILL keeps it around 16-17.
-Notice cam sensor date, car was driven with the bad cam sensor for about 6 months.
-One of the belts had a hose clamp grinding into it so it is slightly worn in the middle. Will replace it sometime soon. Doesn't seem to pose a problem. It was grinding for about a month.
-Car was driven with memory steering for about 1.5 years. Fixed once I did #17.
-Junker oil pressure sensor has died. Car reads over 120 oil pressure constantly again. Learned my lesson and will be putting a new one in in the summer.

----------Current unknown problems/ may be associated with MPG-----------------
1. Weird ringing noise while driving AND in park/neutral. I have not had myself or someone else check the flex plate or transmission for it yet. http://www.bonnevilleclub.com/forum/...hlight=ringing

Note: #23 above, I believed the exhaust pipe to be the ringing problem, but this didn't help.

2. Grinding noise at idle (800 RPM). Lower the RPM, louder it is. Goes away with any acceleration. Possible Harmonic Balancer? Sounds more like its coming from the supercharger, but we rebuilt the drive unit and replaced the oil at the time.

3. Grinding noise that correlates with acceleration, mainly lower rpms. Not present at idle. Seems to make the noise while in park/neutral. This problem was not present with problem #2. So the cause should be separate. I noticed the pulley above the harmonic balancer seems to move left to right a bit while running.

4. Dash display with mileage, car picture, etc. will sometimes flicker. It may even go completely blank. This is rather uncommon and happens more when its cold and/or wet out.

5. Do we have 3spd transmissions? I have watched my RPMs and it only seems to switch to 3rd gear. ALSO - when i'm driving at a consistent speed and my car kicks into overdrive it has a pretty bad bucking to it. Knocking it out of overdrive by more acceleration or deceleration stops it. (2000RPMs....then it ,from what I believe, kicks into overdrive and goes to about 1500RPMs) This hesitation only happens when it kicks down to this 1500RPMs.
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Thats about it boys. I have given you so much information because I want to at least let it show that I have done my homework and am not expecting you guys to do everything for me here

I was thinking cat. converter which i don't mind buying a Magnaflow if I have to. I'd just rather pay for a welding job for it once I have the money for a new exhaust system this summer. It sounds hollow, but its probably the original.

Any ideas for my bad MPG? Or even for my other possible problems if something comes to mind, but i'm mainly worried about my MPG since its constantly costing me money.
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Old 02-04-2008, 02:48 AM   #2
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It sounds like you may have problems with your torque converter clutch. If it'* not locking properly or slipping in and out, it will cause this "bucking". If it'* not locking up, gas mileage on the highway goes down the tubes. Sometimes the trans will not shift to 4th gear either. You should have this checked at a rebutable transmission shop. Many do free test drives and electrical diagnosis. There are other problems that could cause this bucking too.

As for your noise, it sort of sounds like a "front of engine" issue, something belt driven. Take the belts off (very quick and easy) and wiggle all of the pulleys to check for play. Slight play in idler and tensioner pullies is normal. Spin them and feel for roughness. Be sure to check the water pump. Also, grab the outermost circumference of the harmonic balancer and wiggle on it. The rubber that joins the two sections commonly seperates, causing all kinds of noises at lower RPM. Did you replace your supercharger coupler with the nose rebuild? It sounds like you could have a worn coupler. Wiggle the supercharger pulley when you have the belt off. When turning it back and forth you shouldn't feel any "slop" in the pulley before it engages the rotors. If there is any slop, you need a coupler.
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Old 02-04-2008, 09:08 AM   #3
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Thank you for your reply. When I was working on the LIM gaskets I did wiggle the supercharger'* pulley and there was some play before it actually moved the components. We rebuilt the drive unit, but did not do anything with the coupler. Must I buy a whole new unit, just the coupler, or is it possible for me to rebuild the coupler as well? Thank you.

I will have to check the other pulleys this week. A question about the harmonic balancer. So there are essentially 2 pulleys connected together by some rubber? Or the single pulley that is separated by belt contact with rubber?

So we do have 4spd transmissions? I can go from 0mph and just watch my RPMs for when it shifts. The RPMs only shift twice, meaning 3rd gear. It has done this since I have had the car. So you mean to tell me that this thing should be shifting one more time? If the torque converter is the coulprit are we looking at having to take the tranny out or even possibly rebuild it?

If my car has not been shifting into 4th gear and the overdrive bucks, would that explain my bad city mpg?

Yet again, thank you.
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Old 02-04-2008, 09:41 AM   #4
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i have a 95 Ei and i get 15 city milage... BUT, on the highway i would normaly get 22 or 23.. i've been told thats still low but i don't think that a supercharged car will do well in the city anyhow, no matter how you drive. especially if its 95% city...
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Old 02-04-2008, 04:27 PM   #5
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Took the car to a mechanic, but he was rather busy. He told me that I have a 3spd tranny with overdrive so my shifting is fine. The overdive bucking did not happen while he was driving.

He heard the ringing noise and even he was unsure as to where it was coming from.

So I may have a torque converter clutch problem with the overdrive hesitation and that unknown ringing noise, but what else could be my bad mpg? Or is it just supposed to be at that low #
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Old 02-05-2008, 03:04 AM   #6
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I'm not too knowledgeable on the older Bonnevilles, maybe somebody can chime in here? As far as I know, this car should have a 4 speed. Saying "A 3 speed with overdrive" is confusing. This could mean a trans with 3 gears plus a 4th overdrive gear or a 3 speed where the 3rd gear is an overdrive (below 1:1 ratio), which I've never seen.

You can replace the coupler yourself. It'* rather easy. You remove the nose drive from the SC. I think on your car the supercharger may have to come off to remove the nose drive. Again, a Bonneville owner should chime in here. Once the nose drive is off, you pull the coupler off the nose shaft, or the rotor plate if it'* stuck on there instead. It'* got 3 holes in it that slip over 3 prongs in the nosedrive shaft. You'll see what I'm getting at when you're in there. The holes in the coupler are worn, creating the "slop". Along with the new coupler, you'll need sealer to seal the nose drive to rotor plate mating surface again, and 2 bottles of GM supercharger oil to fill it up. Check out www.rollingperformance.com for the parts.

The harmonic balancer is made in layers, if you will. I'm making up all my numbers here, so don't mind them. Picture a big solid pulley, say 6" in diameter. Now imagine the outer circumference is wrapped in and bonded to a 1/2" layer of rubber. Now imagine that rubber layer is covered with another layer of solid metal, a weight, maybe 1 inch thick. Creating a pulley 7.5" in diameter. Why bother with all this? Quite simply, the crankshaft RPM (while you'd never be quick enough to notice this on a gauge) speeds up when a cylinder fires and then slows down again as it turns around to the next firing cylinder. This cylinder fires and the crank speeds up again. The harmonic balancer smooths this out. As the crank accelerates, the rubber flexes and the outer section is "pulled along" in behind like it'* being towed with an elastic band. Eventually it catches up, around the time the crank is ready to slow down again. Now, since it'* connected with rubber (the elastic band) it swings out in front of the centre section of the balancer, and tries to speed up the crank by pulling it around. In this way, it helps to keep the crankshaft RPM more constant.
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Old 02-05-2008, 03:20 AM   #7
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The mechanic told me that the harmonic balancer was fine because I would be able to hear it continue to make noise as soon as I shut the engine off, that wasn't the case.

My water pump pulley has a good amount of play and I will have to replace that. Oh joy, the motor mount has to be removed on that side. Figure i'd do the SC job with the water pump since it may be just a bit easier to replace the pump with the SC off.

So new waterpump, SC coupler, and oil. What would the steps be for me to look at my torque converter clutch?
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Old 02-05-2008, 05:14 PM   #8
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16-18 MPG is also what i get in my SSEi. And i don't have any major problems now.
the only thing i haven't been able to do is change the fuel filter. (it'* rusted in place)
And about half of my driving is highway.
And an interesting thing to note: i always use to get 18 no matter how i drove, with the stock intake
Then i put on the rice pipe and it dropped to between 16 and 18. Of course the acceleration became
far better, but i was expecting an increase. I go between 60-65 on the highway and don't take
off from a standstill, so i'm just as confused as you are!

I don't wanna do this but my only remaining guess would be.....
the color. She'* white.....
DOH!

Rubix
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Old 02-05-2008, 05:53 PM   #9
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When was the last time the transmission fluid was flushed and the filter was changed? It should be flushed and the filter should be changed every 15-25k. Although it might be too late, this doesn't sound like a common 60 series failure.

What is the appearance of the transmission fluid, and is it the correct level? Bright red? Does it smell burnt?

You don't have a 3 speed. You have a 4T60E-HD, 4 speed, transverse, series 60, electronic, heavy duty with overdrive.
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Old 02-05-2008, 08:56 PM   #10
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The fluid is red, doesn't smell burnt, and is of correct level. The filter and fluid has not been changed by me, and I have no idea when it was last changed.

So I have a 4-spd transmission? Then according to me watching my RPMs from 0-45mph, it never gets into 4th gear.

UPDATE: The overdrive problem is not an overdrive problem. It happens while in regular drive as well. Driving at around 35-40mph, 2000RPMs. The car will suddenly drop to 1500RPMs and will start bucking. If I hit the brake it returns to 2000RPMs and runs fine, but immediantly returns to 1500RPMs with bucking. The switching of RPMs, thats the torque converter then I take it?

The noises I have concluded to be a bad water pump (pulley is moving left to right) and SC coupler, except for the ringing noise.

I always thought that RPM drop as the overdrive engaging. Obviously not if its doing that in regular drive. How do I distinguish the overdrive engaging?

White car as well No matter how I drive highway/city grandma/demon, I still get the same MPG around 16-18. Mainly was 17, rice pipe with K&N filter makes it more 16 now.
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