Hello fellas. I have two questions When i shut off my motor, it'll tend to "run-on" for a second or two. It always happens when i've driven my car longer than 10 minutes. Sometimes it's excessive and sometimes it shuts off without a problem. What can i do to eliminate this problem? Second question is that when i accelerate, i hear a rattle from the motor. When its idle, and when i decelerate, the noise is non-existant. Could this noise have something to do with the motor running hot, could i be my valves? thanks
The problem of run on occurs when compression in the cylinder alone combined with heat is sufficienct to ignite the remaining raw fuel. I suggest you check your idle speed to see if it can be reduced . Also the timing may be too far advanced and/or you are using too low a grade of fuel. I would also check to see if the distributor is working properly. The pinging can be a sign of detonation which may damage your engine.
Run-on can be cured easily if your car is equipped with an ignition box. MSD even puts the fix in their owner's manuals. You just have to put a diode in the brown wire going to the box on your fire wall. That allows the electricity to only flow one way. I can post the instructions here if you like. Just PM me and let me know. Phil
Thanks fellas, I recently bought the car and all was fine until about a week ago. I checked the timing this morning. It idled fine until the motor warmed up, then the timing mark shot up left of tdc, i'd say close to 5-8 degrees advance. So I retarded it to four degrees before tdc. It still had slight run-on. What grade gasoline would you guys suggest using? I've always used 87grade. Thanks for your help fellas!
If you have a 70 with 10:1 compression you need to put in the premium. Try mid grade if not. May stop the run-on.
Since you posted in this forum, I'm going to assume you have a buick 350. Unless it's one of the few high-compression motors, 87 octane should be just fine. I would check your idle speed, and make sure that it is set properly. A raised idle speed, even only by a little bit, can cause run on. Certain '72 and up models have an idle stop solenoid, which retracts when you turn the motor off, and it allows the throttle blades to close completely, avoiding run-on. When you check your timing, it would be wise to check your total timing too, and see when it is coming in. You can check this using a "dial back" timing light. Generally 34-36 degrees is considered normal. Also check to make sure your advace stop bushing is still in place, if not this will allow the timing to advance way beyond what it is supposed to. If your 350 is equipped with EGR, this sounds like the symptoms of a bad EGR valve, or an engine which had the EGR valve removed and didn't have the carb and timing recalibrated to match the changes. Good Luck!
Yes, it's a 350 with 2barrell. All stock I've filled up with 92octane and left the timing 4degs btdc. I've driven it to work (20 miles) got on the accelerator and the rattle that i've mention in my first post is gone. The motor didn't run-on after i shut it off. I'm thinking that it ran well this morning because outdoor temp is always cooler in the morning. I'll have to wait until my drive home today to see what happens (this is when all my problems normally arise.) It's supposed to be 90 degrees this afternoon.... Thanks for the help everybody! Very much appreciated!
Iwas going to comment but you just discovered your problem, insufficient octane. The run on and rattling or pinging noise is a dea give away. I don't know what year your engine is, it might be a low compression engine, but if there is a lot of carbon build up inside, it will raise the compression as well as cause hot spots. Jim burek
I had this happen on my 350 wagon a year or so back. Discovered my rigged up electric fan provided enough juice while it spun down to run the motor a sec longer. Neat, huh.
I've also heard that run-on could be caused by excessive carbon build-up on the piston heads and valves... An old friend of mine told me that he would warm up the motor real good, then pour a little bit of water in the carb while turning about 2500 RPM. He claims that the cool water would thermally shock the carbon deposits loose and eliminate the run-on.... I don't know if this really works, or if it's just stupid....
So far, the higher octane has worked The motor still has slight run on, but no where near as drastic as before. Rattling is almost non-existent now. I'm concerned about carbon build-up. So i plan on using carb/injector cleaner with the next few fill-ups to finish off the job. P.S. I'm installing a water-temp guage. I have to replace the stock sender because it's for a mechanical gauge. The stock sender is electric and is as stubborn as heck and tends to strip because the metal is so soft. Anyone have any tips that would make removal easier?
not sure if they make them but for anodized parts you use wrenches made of the same material. Problem most fittings are brass and tools are steel. Which make it real easy to over do it. What you need is a brass wrench but not sure if you can find one. I recently got a new 2lb brass hammer and its great that I can beat the crap out of stuff and only the hammer gets messed up.
hehehe....i think the problem is that i cant get a good angle on the sender. I'm working in between the carb, water pump hose, a bracket(for the plug wires i believe). I'm just going to have to get busy and get to know this machine (as i should force myself to do) and remove these items to get this sucker off. My reasoning however is that it's "just a sender, how hard can it be? I shouldn't have to remove these items"
Use what GM intended you to use for this... I'd go to your favorite local GM parts counter and ask for a bottle of GM Top End Cylinder cleaner and follow the instructions on the bottle. Does a great job at cleaning up carbon and only costs about as much as a couple of bottles of expensive injector cleaner. Made by GM, good for Carbs or injection, safe for catalytic convertors, and cleans loads better than water. The clean takes about a half hour, usually requires no tools and anyone can understand the directions. As a precaution, I would plan to change the oil and plugs immediately following a Top End Clean treatment just because it does loosen up so much gunk. I've used it on a couple of cars and it does make quite a difference on a high-mileage motor. My dad and I even used it once on a friend's car that had been sugared in the gas tank and it got the car running pretty good again. - Freed PS- I've heard that using spark plugs that are WAY outside the proper heat range and/or gapped wrong can cause the dieseling you are talking about. If you do the Top End clean and change the plugs, make sure you get the right heat range plug and check the gap.
