[al4.9l]

Hi guys,

i have a 1992 sedan deville 4.9L with about 250,000 miles on the clock. i have nearly finished restoring it as it was due for the scrap heap. Anyway the exhaust had like 9 holes running down it so i replaced the entire exhaust including catalytic converter and silincer and the O2 sensor as well.

Now it runs like a dream but the MPG is down to 10MPG from 15MPG that is just crazy. the DTC that comes up is E44 O2 sensor running lean. i do maninly highway miles and if you turn on cruise control the mpg meter goes slight erractic then the code comes up.


just so you know i have changed the following in the last 2 years:

spark plugs, ISC motor, genrator, Vbelt, idler, battery, HT leads, airfilter, water pump all are AC Delco

many thanks

al

 

[Ranger]

Try cleaning the TB and if the 2 EGR tubes down in it are all carboned closed, rod them out to open them up. A inoperative EGR will hurt gas mileage.

 

[al4.9l]

many thanks Rnager, i forgot to say that i had already changed the EGR as well but i will rod the tubes again can you please tell me what you mean by "TB"?
thanks al

 

[Ranger]

TB = Throttle Body

 

[drewsdeville]

You state in the original post that all symptoms, including the O2 lean code, occur when turning on the cruise control. Check the cruise control servo diaphragm and it's vacuum lines for leaks and proper connection.

 

[cadillac_al]

250,000 miles on the clock. I think all engines wear out eventually and lose some efficiency. It probably has some leaky injectors too. I would check the vacuum to see how normal it is running.

 

[al4.9l]

Hi guys,

i have been looking at the throttle body and i have noticed that there are couple of perished rubber parts. in particular the small rubber hose from the fuel pressure regualator to the TB and the even smaller hose that goes to the transaxle modulator. i have just found out that neither rubber part is available for sale. if i buy a new fuel regualtor it does not come with that rubber hose:

1) does anyone know where i can buy there parts?

2) does anyone know what the transaxle modulator actually does?

i have concluded that the fuel pressure REG is the reason why i have poor MPG, if you disagree pls let me know

thanks

al

 

[Ranger]

A leaky FPR WILL cause lower fuel mileage as it is sucking unmetered fuel into the engine. It sounds like all you need is a length of vacuum hose if I am understanding you correctly. You can buy that by the foot at any parts store.

 

[drewsdeville]

The modulator controls shift firmness/softness, and nothing else.

 

[cpineda714]

what i think could be causing your problem is a leak maybe around the exhaust. Here is why, if the exhaust headers are not seal properly there will be unmetered air going inside the exhaust. The O2 picks up this lean condition due to the leak. The 02 sensor sends a signal to the computer to richen up the mixture to bring back the air/fuel mixture to 14.7 which is the ideal mixture for the safe of the catalytic converter. That's why maybe your MPG is not good because the ECM(obd 1)computer is energizing your fuel injectors more than it should to bring that mixture to proper mix. What you can do is, check LTFT(long term fuel trim) to see if it's with in normal ranges. If it is between 128 to 138 that's consider normal. But if it is above 138, you got a problem. This number means the computer has richen the mixture due to the lean mixture or leaking air. You can also check the ON TIME of the fuel injectors. Or what i do is, get a propane can and start spraying around the exhaust headers or vacuum lines. If you hear the engine RPM increase a bit, you found the leak. Do this while monitoring the 02 sensor voltage. I hope this helps you identify your problem and get your MPG to proper levels.

 

[cpineda714]

I forgot something, it's really important to fix this problem because RICH mixtures(high CO)are the #1 causes of catalytic converters to go dead. here is why, the cat is trying to oxidize(add air)the rich mix for lower emissions. too much CO the hotter the cat gets, causes could melt the inside. ur looking at a brand new cat. expensive by the way.... that's it. bye

 

[drewsdeville]

The OP stated that he already replaced the exhaust due to leaks.

 

[Ranger]

Here is why, if the exhaust headers are not seal properly there will be unmetered air going inside the exhaust.

The exhaust is under pressure. If there is a leak air cannot enter, much like a leak in a garden hose. Unmetered air can only enter via an intake (vacuum) leak.

 

[drewsdeville]

The exhaust is under pressure. If there is a leak air cannot enter, much like a leak in a garden hose. Unmetered air can only enter via an intake (vacuum) leak.

Er, it's my understanding that fresh air will enter the exhaust at a leak. In fact, I've witnessed this on a number of occasions, mainly through exhaust manifold leaks at the mating surface to the head. The bank with the exhaust leak always has a rich biased fuel trim problem.

Due to the velocity of exhaust gases, fresh air is drawn in through the leak. Do you know how a siphon sandblaster works? Sort of like that.

 

[Ranger]

Does not sound logical to me. I've seen (and burnt my finger) on an exhaust manifold gasket leak. Believe me, there was no vacuum there drawing in fresh air. There was down right positive pressure and a lot of it.

 

[drewsdeville]

I know it doesn't sound logical, but the PCM confirms it every time. Richened fuel trim on the leaking bank means the 02 sensor is somehow reading oxygen rich exhaust...

See this link: http://www.ehow.com/list_5746543_effects-cracked-exhaust-manifold.html

There shouldn't be a terrible amount of pressure in the exhaust system, I would think no more than a few psi. Otherwise, it'd be blowing donut and manifold gaskets all the time. Exhaust is made to flow, not restrict. Flow is inversely proportional to pressure

 

[cadillac_al]

I know it doesn't sound logical, but the PCM confirms it every time. Richened fuel trim on the leaking bank means the 02 sensor is somehow reading oxygen rich exhaust...

See this link: http://www.ehow.com/list_5746543_effects-cracked-exhaust-manifold.html

There shouldn't be a terrible amount of pressure in the exhaust system, I would think no more than a few psi. Otherwise, it'd be blowing donut and manifold gaskets all the time. Exhaust is made to flow, not restrict. Flow is inversely proportional to pressure

I wise man once said "you can't believe everything you read". I've always been highly skeptical of everything i read. When I read that link it seems fairly logical yet highly theoretical; almost like something from Ripley's. I'm not raggin' on ya Drew because you are usually full of good info.

My Deville has had an exhaust leak for 4 years. It never affected the fuel mileage. 2 months ago I started getting a code that my O2 sensor isn't ready and now my fuel mileage has gone down quite a bit. I think the O2 sensor is just failing from 195,000 miles. An exhaust leak is a major nuisance but I doubt it will really ruin your engine. These 4.9 manifolds are hard to work on because the bolts or studs are so wimpy that they snap off easily. I don't have access to a lift to fix it properly (yet) so I've had to live with it.

 

[drewsdeville]

The link was merely to support my personal experiences rather than to act as sole proof. If you are curious enough, you can easily replicate it yourself. Induce an exhaust leak ahead of the O2 sensor and view the fuel integrator or block learn values through the onboard diagnostics. You WILL see them increase on the leaking bank only.

As far as ruining your engine, I agree, won't happen. Throwing fuel trims out of whack on fuel injected engines is a definite truth, however.

As far as 4.X manifold studs, your comment is surprising. These engines are some of the few in which I've never broken a stud. I'll even go as far to say as they come out really easy! I find the problem is always the y-pipe-to-manifold bolts :/

 

[Forgoten214]

How many Oxygen sensors are there? I'm getting E13 and E44 myself. Did the OP fix the problem? What was the problem?

 

[Ranger]

I believe the 4.9 only had one O2 sensor.