81 runs really rough until warmed up
Thread Starter
4th Gear
Joined: Mar 2018
Posts: 4
Likes: 0
81 runs really rough until warmed up
I've been combing the forums and it feels like I get close to finding some feedback but nothing is recent enough (links are dead, etc) to yield me advice, so I'm going to shoot this out. If there is an old thread that covers this specifically, please let me know.
My 81, which I purchased last summer and I believe to have about 60k on it (with original ignition parts), runs really, really rough (misses) when I first start it up. It'll die a few times until, after about 10 minutes of getting warmed up, it eventually smooths out and feels like it runs perfectly fine. I have 2 directions I'm heading and I'd like some expert feedback, particularly since this electronically controlled carb is confusing!
1) I've ordered new plugs, wires, coil, distributor cap, and rotor
2) a semi-mechanic friend of mine said "it's a choke problem". On that advice, I found a thread that said the problem is likely the choke, but more specifically the "front vacuum break" which I, for the life of me, cannot find for sale anywhere! I followed the advice on how to check it in the quoted text below and it does appear to be bad (does not hold when I plug the line with my finger). Is it possible to buy one of these? If so, can you please link me to it? If not, what do I do? I can buy a rebuild carb, but I may have an electrical problem.
Some background: all vacuum lines are new and all sensor valves are new. New fuel pump. New EGR valve. I am trying to keep this thing stock so I'd like to limit the feedback to that direction as I already know I'll probably get advice to tear out all the electronics, emissions, etc but I've already spent too much on keeping it stock to do that at this time. I just want her running smoothly!
Here's that quoted text from KENS80V: "Sounds like the vacuum break diaphragm is ruptured. This is the small canister in front of the choke. If the diaphragm (rubber) inside the canister is ruptured the choke will not unload slightly at idle. When the engine is first started (cold), the choke wants to close completely. The vacuum break diaphragm pulls the choke valve to a preset opening once the engine is started. This opening is usually about 1/8 to 1/4 inch. If the diaphragm is ruptured ....no opening will take place. The engine will run rough and want to stall because it isn't getting the required amount of air needed. Once the engine starts to warm up and the choke starts to open....the idle will stabilize. Easiest way to tell if it is working is to start the car when cold...observe the opening (slightly) of the choke valve. If there is no opening of the choke valve 1/8 to1/4 inch and the choke valve remains completely closed then the rubber diaphragm is ruptured. You can test it by removing the vacuum hose to it .....push the arm into it then cover (with your finger) the vacuum port. The diaphragm should hold it in. If it pushes back out while your finger is covering the port then you have found the problem.They are easy to change and not very expensive."
My 81, which I purchased last summer and I believe to have about 60k on it (with original ignition parts), runs really, really rough (misses) when I first start it up. It'll die a few times until, after about 10 minutes of getting warmed up, it eventually smooths out and feels like it runs perfectly fine. I have 2 directions I'm heading and I'd like some expert feedback, particularly since this electronically controlled carb is confusing!
1) I've ordered new plugs, wires, coil, distributor cap, and rotor
2) a semi-mechanic friend of mine said "it's a choke problem". On that advice, I found a thread that said the problem is likely the choke, but more specifically the "front vacuum break" which I, for the life of me, cannot find for sale anywhere! I followed the advice on how to check it in the quoted text below and it does appear to be bad (does not hold when I plug the line with my finger). Is it possible to buy one of these? If so, can you please link me to it? If not, what do I do? I can buy a rebuild carb, but I may have an electrical problem.
Some background: all vacuum lines are new and all sensor valves are new. New fuel pump. New EGR valve. I am trying to keep this thing stock so I'd like to limit the feedback to that direction as I already know I'll probably get advice to tear out all the electronics, emissions, etc but I've already spent too much on keeping it stock to do that at this time. I just want her running smoothly!
Here's that quoted text from KENS80V: "Sounds like the vacuum break diaphragm is ruptured. This is the small canister in front of the choke. If the diaphragm (rubber) inside the canister is ruptured the choke will not unload slightly at idle. When the engine is first started (cold), the choke wants to close completely. The vacuum break diaphragm pulls the choke valve to a preset opening once the engine is started. This opening is usually about 1/8 to 1/4 inch. If the diaphragm is ruptured ....no opening will take place. The engine will run rough and want to stall because it isn't getting the required amount of air needed. Once the engine starts to warm up and the choke starts to open....the idle will stabilize. Easiest way to tell if it is working is to start the car when cold...observe the opening (slightly) of the choke valve. If there is no opening of the choke valve 1/8 to1/4 inch and the choke valve remains completely closed then the rubber diaphragm is ruptured. You can test it by removing the vacuum hose to it .....push the arm into it then cover (with your finger) the vacuum port. The diaphragm should hold it in. If it pushes back out while your finger is covering the port then you have found the problem.They are easy to change and not very expensive."
Pro
Joined: Jun 2016
Posts: 567
Likes: 82
From: Auckland
https://quadrajetparts.com/rochester...ler-p-223.html
Personally I would prefer to check it with a vacuum gauge rather than a rough finger method, but it does sound like this could be your problem.
Personally I would prefer to check it with a vacuum gauge rather than a rough finger method, but it does sound like this could be your problem.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
I am more concerned if the carb is going to FAST IDLE.
If the carb is not going into FAST IDLE...it will die and act all up until it gets warm enough to be able to run at the idle setting that it is set at. And with engine being COLD...this makes it that much worse.
Hopefuly CARE was taken when replacing ANY of the vacuum hoses on this engine.
The TVS' ( temperature vacuum switches) are specific and how the vacuum hoses go to them are important.
I hope the EGR valve that oyu replaced was like the one that was on it due to ...if my memory is correct...there are three different types of EGR valves (P) positive pressure (N) negative pressure and (0) which is basically neural. The original EGR valve will have a (P) or (N) or (0) stamped on the top of it. And I have run into some EGR valves that when the engine was running the exhaust pressure was cauing the pintle of the EGR to raise and cause all types of havoc when the engine was trying to idle. beacsue...there should be NO EGR when idling. I DOUBT that this is your problem due to you stated it runs better when it gets hot. But it was just a thought.
I would be EXTREMELY careful if you or someone goes in and starts to change your curb idle setting...because this can more than likely throw off your TPS setting...thus requiring it to also be re-adjusted.
DUB
If the carb is not going into FAST IDLE...it will die and act all up until it gets warm enough to be able to run at the idle setting that it is set at. And with engine being COLD...this makes it that much worse.
Hopefuly CARE was taken when replacing ANY of the vacuum hoses on this engine.
The TVS' ( temperature vacuum switches) are specific and how the vacuum hoses go to them are important.
I hope the EGR valve that oyu replaced was like the one that was on it due to ...if my memory is correct...there are three different types of EGR valves (P) positive pressure (N) negative pressure and (0) which is basically neural. The original EGR valve will have a (P) or (N) or (0) stamped on the top of it. And I have run into some EGR valves that when the engine was running the exhaust pressure was cauing the pintle of the EGR to raise and cause all types of havoc when the engine was trying to idle. beacsue...there should be NO EGR when idling. I DOUBT that this is your problem due to you stated it runs better when it gets hot. But it was just a thought.
I would be EXTREMELY careful if you or someone goes in and starts to change your curb idle setting...because this can more than likely throw off your TPS setting...thus requiring it to also be re-adjusted.
DUB
Melting Slicks
Joined: Aug 2004
Posts: 2,920
Likes: 229
From: Oakbank Manitoba
Your symptoms were exactly like mine years ago when I used a quadrajet carburetor (since converted to FI). Hope that you have found the problem and I'm glad my quote assisted you.
Rock Auto is selling the choke pull off. https://www.rockauto.com/en/catalog/...+pull+off,6000
Most of the auto parts stores should have it .......Standard Motor Parts CPA141
Rock Auto is selling the choke pull off. https://www.rockauto.com/en/catalog/...+pull+off,6000
Most of the auto parts stores should have it .......Standard Motor Parts CPA141
Last edited by KENS80V; May 5, 2018 at 07:05 PM.
