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73-87 Chevy _ GMC Trucks => Performance => Exhaust => Topic started by: Deathscythe on September 01, 2018, 10:50:15 AM
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Hi all, have a simple (I hope) question. I have an 86 GMC Sierra with a 305/350T. I'm in the process of deleting all the emissions equipment on the truck and will be removing the heat riser as well, once i can pull it out.
Until then, when cold there is an audible ticking or similar sound coming from the passenger side near where the heat riser is. My friend at work says it sounds like a bad push rod or flat cam lobe. But, once it warms up it goes away, making me think it may be a leaking heat riser gasket which I know is a common issue.
So, this morning I went out when the motor was cold, started it and immediately heard the regular ticking noise. I pulled the vacuum line off of the heat riser and it stopped, plugged it back on and it started again.
So my question is this, with vacuum off, is the heat riser valve open or closed?
Until I can pull it and find a suitable spacer, can I just pull the line and drive, or will have have to physically wire the valve into a certain position?
As of now, my plan is to pull it when I can, pull everything out of the valve and welding the holes shut and reinstalling it. A spacer costs around $60, a weld is free.
Thanks in advance for any information.
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Depending on where you live, like cold country, a heat riser is not a bad thing to have, as it gets the intake manifold up to speed quicker, (as in atomizing fuel). Without it, expect doggy cold performance when cold. Now if you live near the Equator, different story.
I can't say for sure, buy my gut says the default position of the riser valve is open (as in open exhaust flow), and the vacuum closes it until the engine is warm.
Having said that, the vacuum is controlled by a valve that reacts to engine temp, which sound like it's the problem, weak vacuum-not holding the valve in place while cold.
While I'm no fan of emission equipment, that is one item I would leave alone for the benefit of cold mornings.
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I have done some research and I believe you are correct. Vacuum keeps it close until warm and slowly shuts according to vacuum.
Now, most of the emissions equipment is being taken out, almost everything is deteriorated to the point of replacement, so I'm just getting rid of it.
I did run across an article that stated that when a heat riser valve is worn out, the valve itself can become loose on the shaft and rattle when partially or mostly closed. I believe that is the issue, maybe in conjunction with a leak. I'll know more once I get it pulled out and inspected.