Rough Idle / lower RPM

[bad neighbor]

Hi all
My truck has been running great for a few months now after locating a rough idle problem and repairing it. Turned out to be a cracked cruse control hose that came off the intake manifold. Had rough idle and raised the rpm. Repaired the hose, idle smoothed out and rpm dropped to normal. Cool.
Now I have a new rough idle, not as bad as last time, and the rpm's have dropped. What the heck?
First thoughts was that I had another bad vacuum hose possibly coming off the top of the carb. Found nothing that stood out as the problem.
So the EGR came to mind, but this is the first engine I have ever had with an EGR so not sure what to expect if they malfunction.
I have the shop manual found on this site, but it is missing the diagnosis sections.
Also checked the PCV valve and appears to be working properly.
The crazy thing about this is overall performance seems to be better. IE I feel a little more power ???
C20 85 Suburban 454 4BBL carb
Thanks for your input

 

[1987 GMC Jimmy]

EGR valves are tricky - trickier than I used to think they are. A clearly bad EGR valve will have a stuck diaphragm with no travel. It is also my understanding that some valves should hold vacuum when the pintle is exposed to exhaust pressure, and some should simply be able to hold vacuum on the bench. I'd also take the valve off and test it as best you can while also cleaning the carbon off the pintle and in the ports in the manifold. A lot of things can leak vacuum, though. I'd get a vacuum gauge to establish a base line and a set of vacuum caps from the parts store and plug everything on the carb and manifold. See if the vacuum reading goes up after that. That should be a good test of whether or not it's a vacuum issue at all. If you find that it is, I'd check the lines, fittings, and actual components related to the evap system, trans vacuum modulator, vac advance diaphragm, EGR, EFE, the check valve for the A/C and cruise port that comes of the manifold (aftermarket ones are cheap and can get little pinholes in them, but they'll whine when they do this), and all the little thermal vacuum and vacuum delay switches. I think cutting it all off and slowly bringing it back online while repeatedly checking your vacuum reading will narrow it down properly.

 

[Tobe's84Jimmy]

the advice from 1987 GMC Jimmy sounds really good. when you fix one prob, sometimes you shake another on loose. good luck

 

[bad neighbor]

Finally got some better weather here so popped the hood and did a little testing.
So here is what I came up with. I do have vacuum coming from the ported vacuum switch to the EGR with warm engine off Idle. No vac at idle. The EGR does operate with a vacuum pump and engine vacuum. It also holds Vacuum.
The only thing I can't test is if it is completely closing or not which might cause the rough idle.

 

[bad neighbor]

Pulled the EGR. Not really what I was expecting design wise. Also looks fairly clean. not too much build up. Did a soapy water bubble test. Blowing air into the square side got a few very small bubbles around the plunger.
Going to proceed with fabricating the block off plate and test without the EGR.

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[Craig 85]

I'm not sure if you have all the ported vacuum switches still on you truck or not. My truck is the same year and I just replaced all the under hood vacuum lines due to them being brittle and soft. You can get various leaks from the lines, especially at the ported switches. I also changed out all the switches too as my truck had been parked since 1998. I just wanted to make sure all the timed vacuum parts worked correctly.

 

[bad neighbor]

Well it wasn't the EGR valve. Engine ran the same with the EGR delete plate installed.
Good news is I didn't break any bolts, the EGR got cleaned and I know I will be able to replace the EGR when needed thanks to anti-seize on the bolts.

Next will be replacing some vacuum lines as Craig 85 recommended.

 

[Honky Kong jr]

Pulled the EGR. Not really what I was expecting design wise. Also looks fairly clean. not too much build up. Did a soapy water bubble test. Blowing air into the square side got a few very small bubbles around the plunger.
Going to proceed with fabricating the block off plate and test without the EGR.

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An ancient relic of a bygone era for sure....

 

[bad neighbor]

Just got done replacing about 20' of old, cracked, dried out 5/32 vacuum hose which was about 15 different pieces. No improvement, but they still needed to be replaced.
So then I sprayed Gumout Jetspray at the carb to manifold gasket and voila the engine smoothed out and RPM can up as long as I kept spraying. Now I just need to figure out how to keep spraying while I'm driving. And where can I get bigger cans of Jetspray??
Kidding. I'm off to procure a gasket.

 

[bad neighbor]

Picked up a Felpro # 60482 carb mounting gasket From O'Reilly's, $9.00. got it installed and got my truck running GREAT again. Topped it off with a Felpro air cleaner gasket # 60038 and a Wix fuel filter # 33052. Surprised and pleased to see Made in USA on both Felpro. gaskets. Too bad the Wix was not
Thanks for the input

 

[1980 Big Ten]

Picked up a Felpro # 60482 carb mounting gasket From O'Reilly's, $9.00. got it installed and got my truck running GREAT again. Topped it off with a Felpro air cleaner gasket # 60038 and a Wix fuel filter # 33052. Surprised and pleased to see Made in USA on both Felpro. gaskets. Too bad the Wix was not
Thanks for the input

Just had a 403 Olds rebuilt and the builder said to only use Wix filters from now on. Thought you might want to know that you made a solid choice.


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