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Username Post: Power loss problem        (Topic#351521)
PLS 
"5th Year" Silver Supporting Member
Posts: 1058
PLS
Loc: Smyrna, Georgia
Reg: 06-07-12
06-11-18 02:51 PM - Post#2736621    

I realize that the question that I’m asking is concerning a GMC not a Chevy PU and if this is not appropriate I will understand if it is removed. I have a 1998 GMC 1500, 4.3L and for the last 2 to 3 weeks I have experienced a loss of power when going up a hill. On one particular hill I can start out holding the accelerator on a steady 50mph and it will continue to drop until it drops into a lower gear then right back out and pressing the accelerator more does nothing except maybe drop it into a lower gear once again. It hasn’t been tuned up in quite a while and after checking into what may cause this condition it seems that the parts that would be changed in a tune up could cause this problem. I have another problem that may be associated and that is before it began to run this way I had to add coolant to the reservoir because it was almost empty and I had to refill it again last week, that’s about a gallon. There is NO water leak visible ANYWHERE, I have checked the dip stick and only oil to the full mark. Would anyone have any ideas of how to or what to check to see where the water is going and if you think that this could be part of the power problem. I’m going to do a tune up no matter but I am concerned that I may have a more serious problem with the water. Thank you in advance, Lamar



 
Shepherd 
Frequent Contributor
Posts: 1364

Loc: Lake George, NY
Reg: 11-11-15
06-11-18 05:18 PM - Post#2736631    
    In response to PLS

Loss of power can be many things, low fuel pressure, clogged fuel filter, cat failing for starters. Before you do anything, track down the coolant loss issue, start sith a cooling system pressure test.



 
thx1138v2 
Contributor
Posts: 149

Reg: 05-01-14
06-11-18 05:59 PM - Post#2736639    
    In response to PLS

Probably two separate problems. I've had both of them on a '94 GMC Sierra C1500 4.3L.

I had the same water/coolant problem. It turned out to be a bad intake manifold gasket. The water had a slow leak while under pressure. At the front of the passenger side manifold port. That front port is a water port, hence no water in the oil. The water would leak down the front of the head into a pocket in the water pump casting and then evaporate from there so none on the ground and the only way to see it was to pull the A/C compressor and mounting bracket. I found it by using a lender coolant system pressurizing pump from an auto parts store. I'd pump it up while cold and it would slowly drop to zero over about 10 to 15 minutes.

I had to pull the TBI, and manifold and replace the intake gaskets. I used Felpro gaskets. It also gave me a good time to rebuild the TBI and check for cracked/leaking vacuum hoses. I also cleaned some miscellaneous stuff like the MAP sensor, throttle position sensor, etc.

Leak

The red arrow points to the source of the leak. The yellow arrow points to a place that looks like a crack but is not. Green shows the water flow.

I also had exactly the low power problem you are describing. It turned out to be the catalytic converter. The canister inside the converter that holds the platinum pellets somehow came loose and vibrated around until it was 90* in the pipe so it was blocking the exhaust outlet.

The easy way to test for it is to start the truck after letting it cool down all the way and put your hand over the exhaust pipe outlet. You won't feel much pressure - no where near what it should be. You might use a buddy's truck for comparison.

I wasn't exactly sure that was the problem when I found it so I knocked the canister out of the housing with a 3 foot steel rod and a ball peen hammer and bolted it back up. That was it - no more loss of power. I was broke at the time so I just left it out until I could afford a new one and put it in later. It drove like a charm either way.



Edited by thx1138v2 on 06-11-18 06:02 PM. Reason for edit: No reason given.

 
PLS 
"5th Year" Silver Supporting Member
Posts: 1058
PLS
Loc: Smyrna, Georgia
Reg: 06-07-12
06-11-18 09:12 PM - Post#2736668    
    In response to thx1138v2


  • Shepherd Said:
Loss of power can be many things, low fuel pressure, clogged fuel filter, cat failing for starters. Before you do anything, track down the coolant loss issue, start sith a cooling system pressure test.


Shepherd, first of all thank you for your reply. Since you mentioned some of the potential problems let me back up a bit. My fuel gage had been doing weird things for 2 or 3 years and I had thought about changing the sending unit but since I really don’t drive the truck that much, about 4500 miles a year, I just put it off and would keep the tank full. When it began to lose power, I started checking into it and found that the sending unit and the fuel pump was a unit and thought that maybe the pump was going bad as well and that was causing some of the problem so two weeks ago I had a new GM pump/sending unit installed along with a new fuel filter but this didn’t help the problem at all. Then you mentioned the catalytic converter, well guess what about 3 or 4 months ago I began to detect a rattle and could never really pin point it because at idle and in park it didn’t rattle so when I took it to have the tires rotated and balanced I ask the guy if he would check and see if he could tell where the rattle was coming from and he said that it seemed to be coming from the CAT. Thanks again, Lamar













  • thx1138v2 Said:
Probably two separate problems. I've had both of them on a '94 GMC Sierra C1500 4.3L.

I had the same water/coolant problem. It turned out to be a bad intake manifold gasket. The water had a slow leak while under pressure. At the front of the passenger side manifold port. That front port is a water port, hence no water in the oil. The water would leak down the front of the head into a pocket in the water pump casting and then evaporate from there so none on the ground and the only way to see it was to pull the A/C compressor and mounting bracket. I found it by using a lender coolant system pressurizing pump from an auto parts store. I'd pump it up while cold and it would slowly drop to zero over about 10 to 15 minutes.

I had to pull the TBI, and manifold and replace the intake gaskets. I used Felpro gaskets. It also gave me a good time to rebuild the TBI and check for cracked/leaking vacuum hoses. I also cleaned some miscellaneous stuff like the MAP sensor, throttle position sensor, etc.

Leak

The red arrow points to the source of the leak. The yellow arrow points to a place that looks like a crack but is not. Green shows the water flow.

I also had exactly the low power problem you are describing. It turned out to be the catalytic converter. The canister inside the converter that holds the platinum pellets somehow came loose and vibrated around until it was 90* in the pipe so it was blocking the exhaust outlet.

The easy way to test for it is to start the truck after letting it cool down all the way and put your hand over the exhaust pipe outlet. You won't feel much pressure - no where near what it should be. You might use a buddy's truck for comparison.

I wasn't exactly sure that was the problem when I found it so I knocked the canister out of the housing with a 3 foot steel rod and a ball peen hammer and bolted it back up. That was it - no more loss of power. I was broke at the time so I just left it out until I could afford a new one and put it in later. It drove like a charm either way.

thx1138v2, thank you for your very lengthy reply as well and the picture which details where the leak may be. This gives me a place to start looking for coolant loss which I will look tomorrow and I will try to borrow a pressurizing pump from Advance Auto and see how the pressure does. After I look at the catalytic converter tomorrow I may have to find out exactly how you removed the canister from the converter. I’m not worried about the inspection because if you are 65 and older and drive less than 5000 miles a year we don’t have to have it inspected to get a license plate/sticker. I will also try the exhaust pipe outlet test. Thank you very much and I will let you know what happens, Lamar








 
PLS 
"5th Year" Silver Supporting Member
Posts: 1058
PLS
Loc: Smyrna, Georgia
Reg: 06-07-12
06-12-18 07:45 AM - Post#2736703    
    In response to thx1138v2

Checked the catalytic converter this morning and it seems to be one unit from the headers to the flange behind the converter. It looks like you would have to loosen the exhaust manifold flanges and remove the bolts on the flange between the converter and the muffler in order to accomplish what you did to yours, is this correct? I haven't checked the pressure yet, trying to locate a pump. I haven't checked the exhaust discharge waiting to see how the pressure check turns out before getting the water hot.



 
PLS 
"5th Year" Silver Supporting Member
Posts: 1058
PLS
Loc: Smyrna, Georgia
Reg: 06-07-12
06-12-18 12:50 PM - Post#2736729    
    In response to PLS

Shepherd and thx1138v2, I just want to say thank you verrrry much for your help!! I got a loaner pump from Advance Auto and put 10# on the system and thankfully it was just a tiny leak in the radiator just below the upper hose and I will probably try some JB Weld on it. I also order a direct fit catalytic converter from Advance as well and hopefully it will be here by the weekend. As far as checking the exhaust discharge I really couldn’t tell any different in mine and a friend of mines truck. My main concern now is getting the nuts off the studs without doing anything major. Any suggestions will be helpful here. Again, thank you, Lamar (WHAT A FORUM!!!!!!!!!!!!)



 
bowtie44s 
Valued Contributor
Posts: 3988
bowtie44s
Age: 35
Loc: wv
Reg: 08-29-12
06-12-18 12:52 PM - Post#2736730    
    In response to PLS

Change the intake gasket and check fuel pressure.

Jeff

'88 Chevy K3500, aluminum head roller cam 511in³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 
thx1138v2 
Contributor
Posts: 149

Reg: 05-01-14
06-13-18 04:36 AM - Post#2736804    
    In response to PLS

  • PLS Said:
Checked the catalytic converter this morning and it seems to be one unit from the headers to the flange behind the converter. It looks like you would have to loosen the exhaust manifold flanges and remove the bolts on the flange between the converter and the muffler in order to accomplish what you did to yours, is this correct? I haven't checked the pressure yet, trying to locate a pump. I haven't checked the exhaust discharge waiting to see how the pressure check turns out before getting the water hot.


That's correct on the catalytic converter. I used a 1/2" X 3' steel rod through the flange and knocked the canister out the other end. But first check that it is turned sideways in the pipe. You said in a later post the pressure at the tail pipe wasn't noticeably different so that may not be the issue.

Be careful at the exhaust manifold flanges. The studs will be rusted into the manifold if you haven't removed then before. You can put some PB blaster on the top of the studs and let it soak into the stud threads overnight before attempting to loosen the nuts on the other side. Otherwise, you may break off a stud. Or put it on the nuts if possible.

When I replaced mine I replaced the entire unit from the exhaust manifold to the exhaust pipe before the muffler.



 
PLS 
"5th Year" Silver Supporting Member
Posts: 1058
PLS
Loc: Smyrna, Georgia
Reg: 06-07-12
06-16-18 01:32 PM - Post#2737184    
    In response to thx1138v2

Got my direct fit catalytic converter on Friday and got it installed today in about 1 ½ hours but last week while I was waiting on the converter to get here I applied PB Blaster to all of the nuts. I would try and break them loose and nothing, applied it again and finally after about three times and using an impact wrench got all of the nuts off except for two on the passenger side tube to exhaust manifold. I am well pleased with everything as far as the converter goes. It cost a little bit more than just a converter but it is pretty much just remove the 8 nuts and 3 sensors remove and replace. I didn’t want to put too much on the studs and break any of them and since the one that I bought came in two pieces I just cut the passenger tube off close to the first bend and used it instead of the new one and it worked great. The new one came with four o2 sensors openings and I tried to find a bad o2 sensor to plug the extra hole but no luck so I bought a 18mm oil pan plug and worked like a charm. I am going to cut into the old converter next week to see just what was going on inside. It really runs good now and later I will do a good tune-up and take care of the radiator leak as well. I just want to say thank you to everyone who helped me with this project, what a GREAT site with help from all over the world. Lamar

Attachment: IMG_4883.JPG (74 KB) 3 View(s)




Attachment: IMG_4893__3_.JPG (559.16 KB) 2 View(s)






 
PLS 
"5th Year" Silver Supporting Member
Posts: 1058
PLS
Loc: Smyrna, Georgia
Reg: 06-07-12
06-18-18 10:19 AM - Post#2737371    
    In response to PLS

I opened up the catalytic converter this morning and the pictures show what I found. Not sure what position the piece circled should be in but this is the way it was when opened. I didn’t force it but it would move very easily but I couldn’t reposition it and when I moved it around it would replicate the rattle that I could hear before replacing it. The piece that is circled is almost completely blocking the outlet hole of the converter. Open enough to be able to drive when there wasn't much load on the truck but not enough for when it was under a load.

Attachment: IMG_4903_-_Copy.JPG (1.02 MB) 2 View(s)




Attachment: IMG_4906.JPG (879.7 KB) 2 View(s)


Not sure if this is the position that this piece should be in but it looks like that it is blocking the outlet hole of the converter.




 
bowtie44s 
Valued Contributor
Posts: 3988
bowtie44s
Age: 35
Loc: wv
Reg: 08-29-12
06-18-18 11:52 AM - Post#2737377    
    In response to PLS

That was definitely your problem. Don't throw it away. It's worth $40.

Jeff

'88 Chevy K3500, aluminum head roller cam 511in³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap


 
rockfangd 
"5th Year" Gold Supporting Member
Posts: 2454
rockfangd
Age: 32
Loc: Utica ny
Reg: 04-13-10
06-18-18 07:41 PM - Post#2737433    
    In response to bowtie44s

yup it is broken in half and slowly blowing out of the tailpipe as it restricts the flow.
Will run much better now

Old School GM fan FOREVER


 
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