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Discussion Starter · #1 ·
I am an old school mechanic (points and carb era) and I have a 98 5.2 with 175K on it. Has been pretty well kept and I am second owner and have greatly enjoyed this truck. Was heading down the highway the other day and all of a sudden I lost power and the engine started to make a muffled knocking / thumping (not tinny or metallic) noise if I tried to go much above an idle. NO CEL. Temp fine and Oil pressure in the middle of the guage. Pulled it over into a convenient Checker auto and borrowed their scan tool just to see if there was anything but no codes came up. I then pulled the plugs and while they were out borrowed their compression tester and did a quick comp test. Tested 3 cyls and they were decent but the tester hose popped off so I couldnt finish. Cleaned the plugs and drove the truck home 2 miles at 5-10 mph. Looked it over, couldn't find anything else wrong and after using a stethoscope decided the knock was low in the engine and it ran but felt like it was retarded and it sounded like it had baseballs in the pan. Again no codes. Figured it was FUBARed so I parked it but I need to get her back on the road. I have my 84 3/4 T Ram 4X$ on CL and a guy called to discuss buying it and we got to talking small blocks yadda yadda. Heres were it got interesting. He mentioned that he had had troubles of a similar nature when the Knock Sensor went out on one of his trucks and it was knocking because the computer was misfiring. Now especially since it has great oil pressure and no heating problems I am wondering it it could be something as simple as this? I have also seen a couple of posts regarding the crank sensor on the back of the block. Any thoughts on what this might be? I want to eliminate all the other items before I get busy with putting a new engine it this thing. Thanks for any and all help / advice. David in Cave Creek.
 

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Any chance the exhaust is suffering internal failure? any flow at the tailpipe(s)?? I have had mufflers fail, after hitting a puddle, when they're hot, Just a guess, Cat failure is also possible, I guess...
 

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Discussion Starter · #3 ·
Update 01/31 5.2 Magnum

Just wanted to post my progress on running this problem to ground. The more I think about it the trouble must be ignition/timing related. Truck has too much oil pressure and no temp issues for it to be anything in the lower end. The Cat was replaced about 20 K ago and I did a visual inspection today of the entire exhaust. While I was at it I swapped the crankshaft sensor for a known good used one. Only took 2 hours and a pint of blood. Whatta bitch! Anyway, no good news, truck wouldnt even start. It tried to fire on ether but wouldnt start. No codes present either but the battery has been disconnected for a while. Next I pulled off the distributor cap and rotor, the cap contacts inside are pretty scuzzy but nothing terrible like missing chunks etc. Rotor same. I am debating next step. So are there any known issues similar to mine? Like I said, without a code to follow up I am shooting in the dark here. Tomorrow I will put the new cap and rotor on and then try to fire it again. Probably should drain and refill gas too while I am at it. Its at least 6 months old, I will put in the old Chevy work truck with the 292 6, it will run on just about anything! Well back to the drawing board. Again any help / suggestions would be greatly appreciated.
 

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Discussion Starter · #4 ·
Update 01/31 5.2 Magnum

Just wanted to post my progress on running this problem to ground. The more I think about it the trouble must be ignition/timing related. Truck has too much oil pressure and no temp issues for it to be anything in the lower end. The Cat was replaced about 20 K ago and I did a visual inspection today of the entire exhaust. While I was at it I swapped the crankshaft sensor for a known good used one. Only took 2 hours and a pint of blood. Whatta bitch! Anyway, no good news, truck wouldnt even start. It tried to fire on ether but wouldnt start. No codes present either but the battery has been disconnected for a while. Next I pulled off the distributor cap and rotor, the cap contacts inside are pretty scuzzy but nothing terrible like missing chunks etc. Rotor same. I am debating next step. So are there any known issues similar to mine? Like I said, without a code to follow up I am shooting in the dark here. Tomorrow I will put the new cap and rotor on and then try to fire it again. Probably should drain and refill gas too while I am at it. Its at least 6 months old, I will put in the old Chevy work truck with the 292 6, it will run on just about anything! Well back to the drawing board. So what else is there to check that controls the timing and spark?? Could this be an issue with the computer? Again any help / suggestions would be greatly appreciated.
 

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Magnums do not have knock sensors.
Have you replaced the ignition pickup in the distributor?
Have you determined that the intake manifold plenum gasket is sound? When they fail they cause a myriad of problems.
 

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Check fuel pressure. Should be 45 lb. at any time while its running might be a clogged fuel pump filter. Bad thing is its built in fuel pump.
 

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Check fuel pressure. Should be 45 lb. at any time while its running might be a clogged fuel pump filter. Bad thing is its built in fuel pump.
^exactly what I was going to say^

Get someone in the cab to spin it and try a can of starting fluid.

It sounds to me like the fuel pump. Low fuel pressure will cause it to misfire, backfire in the intake, and make some pre-ignition noise.
 

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Discussion Starter · #8 ·
Update 02/01 5.2 Magnum knocking

Hey Gen1dak,
Yah I found out today about the knock sensor. Since I had one handy, I replaced the crank sensor with a known good one but I have not replaced the ignition pickup yet. How bad a job is that? I have an extra distributor with a known good unit in it I could try swapping that out. Besides the crank sensor and ignition pickup what other parts have a bearing on the timing (besides the computer)? I am more firmly convinced than ever that this is an ignition misfire issue. I also have a spare coil so I can try swapping that as well. At least the coil is easy enough, LOL. I just hope it is not a computer issue, last I heard they are still expensive.

So on the fuel side I replaced the FP about a year ago with a OEM type unit from NAPA, but I guess its within the realm of possibility that the pump is not working properly. I definitely should test the fuel pressure tho just to scratch it of the list. Any suggestions as to how to best check the pressure? I tend to think the FP is fine but I guess I need to scratch it off the list. Regards, David & Daisey.. TTYS
 

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There's a port on the fuel rail on the driver's side, looks like a big shrader valve like on a tire. If you can borrow a fuel pressure gauge it's almost just like checking the pressure on your tires...except it screws on.

I've seen new fuel pumps go bad in a week, so it's not uncommon.
 

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The ignition pickup is held in place by a couple screws. It lives under the rotor in the distributor. In the LA engines, it's an ignition pickup, but in the Magnums it acts like a cam sensor, which is key to proper injector sequencing. You could be firing right on ignition, but if the injectors aren't, well you get the idea. Absent that signal, you won't get fuel pressure either, because the computer won't energize the circuit. It's a tedious PITA changing one while the distributor is still in the engine/truck. Computer would be last on my list.
 
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