Rumbling noise in exhaust when accelerating

[NotEnoughGPs]

I picked up a '03 GTP, with 187,000 miles. Car has received a good bit of service from previous owner, but still needs more attention to fix some issues.

 

The entire exhaust is original, but the resonator has been replaced with a straight pipe. I have had the car on jack stands and have looked around the exhaust and it is hanging normally from the rubber hangers. Since I bought the car, I can hear a rumbling or roaring noise coming from the exhaust. It is only when I press on the accelerator to climb a hill, or when on a straight stretch of road, but much more pronounced when climbing a hill.

 

No noise coming from exhaust at idle or when coasting. I can distinctly tell that this noise is coming from the exhaust, not the engine. 

 

Also, I have an engine "no-start" condition that comes and goes. At times, the engine won't start when completely cooled down. Other times, If I keep cranking the engine, it will start. But then after complete warm-up, if I turn it off, it will not restart. But If I let it sit for 20 minutes or so, it will start. At other times, I have tried to start it numerous times in a day and it will not start. The engine cranks freely, but will not start up.

 

Any time I can get the engine to start, it runs well. No stalling and no "check engine light" is on. But there is an exception to this: One thing I have noticed is that after driving for 10 miles or more, the boost gauge will be registering boost at the half-way or full amount on the gauge, but I will barely be touching the gas pedal. When this happens, the idle has become rough and the engine seems like it wants to stall. 

 

Obviously, due to this, I am not driving the car very far, just on short drives to test everything.

 

What do you think is going on? 

[digitaloutsider]

Are you positive that none of the heatshields, particularly the one by the fuel tank, are not touching the exhaust somewhere?

 

The "boost gauge" on the DIC receives it's input from the MAP, which could explain your no-start/rough idle/stumbling issue. Unplug the MAP (red plug, back of the blower) and see if that problem goes away. If it does, there's your problem.

[Imp558]

Some of them are a white plug. Later or replacement are orange. Three pins that look a bit like gun barrels when unplugged.

[NotEnoughGPs]

About the exhaust and the possibility of it hitting a heat shield, that was the reason I removed the wheels and put it on jackstands. I wanted to be sure that the exhaust was not touching anything like a heat shield because that is exactly what it sounds like. Besides, the entire exhaust was removed in order to replace the resonator, so someone who reinstalled it may have not done it right. But when I had the car in the air,  I did not see that the exhaust was touching anything.

 

Someone mentioned that being that the catalytic converter was possibly the original one, and they have a life around 150,000 miles, it is plugged. That could explain why after driving for 10 miles at highways speeds, the boost gauge starts to register boost, and also the car has little power. 

I have an entire exhaust from another GP, so I plan to connect that one to see if that helps. 

 

And I have been alerted to the MAP sensor also. I did not know that the boost gauge gets the input from it. Again, I have another working one (not brand new, but has less than 60,000 miles) and I plan to swap it in.

 

I will work on these 2 items in the next day or so.

I will write back to let you know what happened.

[Imp558]

Check motor mounts. Allowing the engine to rock and changing the trajectory of the exhaust.

[Nas Escobar]

About the exhaust and the possibility of it hitting a heat shield, that was the reason I removed the wheels and put it on jackstands. I wanted to be sure that the exhaust was not touching anything like a heat shield because that is exactly what it sounds like. Besides, the entire exhaust was removed in order to replace the resonator, so someone who reinstalled it may have not done it right. But when I had the car in the air,  I did not see that the exhaust was touching anything.

 

Someone mentioned that being that the catalytic converter was possibly the original one, and they have a life around 150,000 miles, it is plugged. That could explain why after driving for 10 miles at highways speeds, the boost gauge starts to register boost, and also the car has little power. 

I have an entire exhaust from another GP, so I plan to connect that one to see if that helps. 

 

And I have been alerted to the MAP sensor also. I did not know that the boost gauge gets the input from it. Again, I have another working one (not brand new, but has less than 60,000 miles) and I plan to swap it in.

 

I will work on these 2 items in the next day or so.

I will write back to let you know what happened.

 

You mentioned it's more pronounced on hills. I know this may sound unrelated but how is your suspension? If you need new struts, the weight of the car bouncing around may make the pipe hit the heat shield. Also, depending on how loose it is, considering that the whole thing was molested could cause it to shift around. You should shake the pipe and see how much free movement it has. It may sound silly, but I had that issue when I had a test pipe in place of the cat on my car and on some hills, it would rattle. It also hated speed bumps.

 

As far as the cat being original, those things are not maintenance items. They're supposed to last the lifetime of the car, however depending on rich the car has ran in its past, it may not be working as efficient.This was the case with my Cutlass Supreme when I took the cat off. Flakes of the material inside would come off and litter the ground. It was pretty bad. The Camaro I had was worse, the whole honeycomb was loose. Both of those cars ran rich at one point. Moreso the Cutlass since it had a bad LIMG for the longest.

 

Anyways if your car has little power at a certain speed, it would not be the catalytic converter. When those fail, they act as a restriction to the exhaust to the point that the engine will not start because it can't breathe. I would look at the sensors. TPS may need replacing (disconnect it and see if there's any difference), MAF and IAT may need cleaning or replacing and it wouldn't hurt to clean the IAC as well. 

[NotEnoughGPs]

You mentioned it's more pronounced on hills. I know this may sound unrelated but how is your suspension? If you need new struts, the weight of the car bouncing around may make the pipe hit the heat shield. Also, depending on how loose it is, considering that the whole thing was molested could cause it to shift around. You should shake the pipe and see how much free movement it has. It may sound silly, but I had that issue when I had a test pipe in place of the cat on my car and on some hills, it would rattle. It also hated speed bumps.

 

As far as the cat being original, those things are not maintenance items. They're supposed to last the lifetime of the car, however depending on rich the car has ran in its past, it may not be working as efficient.This was the case with my Cutlass Supreme when I took the cat off. Flakes of the material inside would come off and litter the ground. It was pretty bad. The Camaro I had was worse, the whole honeycomb was loose. Both of those cars ran rich at one point. Moreso the Cutlass since it had a bad LIMG for the longest.

 

Anyways if your car has little power at a certain speed, it would not be the catalytic converter. When those fail, they act as a restriction to the exhaust to the point that the engine will not start because it can't breathe. I would look at the sensors. TPS may need replacing (disconnect it and see if there's any difference), MAF and IAT may need cleaning or replacing and it wouldn't hurt to clean the IAC as well. 

The exhaust still makes the noise, and it does sound like it is hitting a heat shield or something. No matter, I have a 2.5" DP and a fairly new complete exhaust which I plan to install.

I am getting no CEL. Not sure if a plugged cat would set one. 

In this GTP, I can't get it to start after the engine is at operating temps. Not sure if the cat is involved, but replacing the entire exhaust will tell me if that was the issue.

About this non-start issue, I have a lengthy post on "POWERTRAIN" forum.

After the car is driven 15 or more miles, I have boost showing on the gauge even at idle. It is when this happens that the car seems to lose power. Also, I had it out this morning, and the fuel mileage is terrible. I reset the MPH gauge, and even after coasting down hills and taking it very lightly on the accelerator, I was getting 13MPH. In my other GTP's, I would be getting at least 26MPH on similar roads. And if I turn off the engine, it will not restart for awhile. Then after trying numerous times for 20 minutes, I can finally get it started.

I have removed and cleaned the TPS as part of general maintenance I am doing to the car. It was pretty cruddy. I have had no trouble with the idle, but the IAC was also carboned up pretty good. I clean that real good. The MAF is a known good OEM MAF that worked like a charm in my other GTP. And the IAT did not look the least bit dirty. 

The most pressing issue is the non-start condition right now. IF I can't trace down why it won't start on a warmed up engine, then I can't drive it very far.

There still is no CEL on when I drive. I believe this issue is vacuum related.