Just bought a 1995 GMC C1500

[VAL]

I see you already got your header gaskets when you make the switch. Decided to leave the PS bracket off? So have it take it for a drive yet to see how good everything else is yet?

I put the bracket back in it place like it was, I had to reuse the original bolts for that. Put in a set of gaskets on it, when I pulled off the old cracked manifolds, they had no gaskets on either side. Im going to get some new gasket rings, I don't know what their called, to hook up the Y pipe to them. After spending the afternoon yesterday, putting everything back together, the ps, belt, fan guard, inner fender and all kinds of misc **** back on, it was late. So I didn't even get a chance to start it up. Unbelievable all the **** in had to remove, to get to that PITA broken bolt.

 

[VAL]

So, I got the truck running with zero exhaust leaks, and it's running very good. Has a little rough idle, but maybe it's just the bad gas mixed in with the new gas in the tank. I got it on the freeway today, was doing 70mph no problems at all. I do however, got this rattling noise going on, only when in gear. What does it sound like it might be? Any suggestions where to start looking? Here's a video I took.

https://youtu.be/iDZfe5poRD8

 

[Camar068]

could it be a cover of some sort underneath?

 

[HotRodPC]

Did you move the exhaust pipes at all during the manifold swap? Maybe broke an exhaust pipe hanger? Heat shield over the converter? Check your engine and trans mount to make sure the tailshaft of the transmission isn't kicked off center etc.

 

[VAL]

Did you move the exhaust pipes at all during the manifold swap? Maybe broke an exhaust pipe hanger? Heat shield over the converter? Check your engine and trans mount to make sure the tailshaft of the transmission isn't kicked off center etc.

I'm gonna check all that tomorrow. But I did crawl under there, and I'm fairly certain its coming from the inside of the cat converter. But why would it do it only in gear, is my question? I put my hand on the guard plate, and it feels like something is loose in there, plus the rough idle, and its extremely hot.

 

[HotRodPC]

I'm gonna check all that tomorrow. But I did crawl under there, and I'm fairly certain its coming from the inside of the cat converter. But why would it do it only in gear, is my question? I put my hand on the guard plate, and it feels like something is loose in there, plus the rough idle, and its extremely hot.

It is certainly possible it can be the Cat, especially if it's a double wall converter and it probably is. Why only at idle in D? Cuz the RPM is bit lower, a little more strain on the engine causing a bit more rougher idle to create the just right condition for the rattle.

 

[VAL]

It is certainly possible it can be the Cat, especially if it's a double wall converter and it probably is. Why only at idle in D? Cuz the RPM is bit lower, a little more strain on the engine causing a bit more rougher idle to create the just right condition for the rattle.

I'm gonna take it to the exhaust shop in the morning for a new cat. Like you mentioned earlier in the thread, the bad cat is probably responsible for the cracked exhaust manifolds also.

 

[HotRodPC]

It is certainly possible. It can be the cat clogged, overheated, caused the extreme hot exhaust manifolds causing them to crack and break. More drive time on the bad converter, sitting and rusting away, eventually blowing out the innerds or at least enough to break a path through the converter or rust through the secondary wall then causing all this rattling in the bad converter. Hopefully that's it. If so, it should run even better with a new converter and less restriction.

 

[1987 GMC Jimmy]

You really came out good on this truck. Just the fact that you're already taking it out on the freeway says a lot. In looking ahead towards summer, if the engine's cooling right, and your A/C works, you'll be in good shape. That is if having cool air is a priority for you. Some people can handle it. The high relative humidity in the South in conjunction with the high temperatures make having air pretty important to me.

 

[VAL]

My old cat! WTF!

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[1987 GMC Jimmy]

That's not good.

 

[HotRodPC]

My old cat! WTF!

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BINGO !!! Exactly what I had highly suspected from the very beginning when I seen that broken manifold. So that was the rattling noise too in D idling too wasn't it?

 

[HotRodPC]

I'd be willing to bet at one time or two, those manifolds got flaming red glowing hot. That converter got hot too to crack and break that catalyst like that. You can see where it was clogged.

 

[VAL]

Yup, you called it! The rattling is completely gone. I still have somewhat of a a rough idle, but I have an O2 sensor, cap & rotor, and new plug wires coming in from RockAuto Auto. Hopefully it'll cure that. Meanwhile, I'll be driving it around some, to uncover the next surprise.

 

[HotRodPC]

Yup, you called it! The rattling is completely gone. I still have somewhat of a a rough idle, but I have an O2 sensor, cap & rotor, and new plug wires coming in from RockAuto Auto. Hopefully it'll cure that. Meanwhile, I'll be driving it around some, to uncover the next surprise.

A tune with cap, rotor and plug wires will most likely do it well. It probably wouldn't hurt to fill that 25 gallon tank and put a 1/2 can of SeaFoam in the gas either. That should help clean up the fuel injectors and the fuel system a bit.