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Idle Missfire

Old 05-28-2019, 09:17 PM
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Default Idle Missfire

Back again with my 1988 Oldsmobile Delta 88, The car has 108k miles and has developed a misfire at idle. This problem started about 3 months ago and has slowly been getting worse. The funny thing is when you put the pedal down there is no hesitation at all, the car takes off fast and drives fine. The car has had a tune up with new NGK Iridium Plugs, and AC Delco Wires. The AC Delco D555 Coil packs are 3 years old. The Fuel Injectors are brand new "Bosch Generation 3" which made a huge difference in power, but no change in the missfire. The 02 Sensor is also a new Bosch part. Another thing I notice is when you snap the throttle down to the floor fast in park the car will rev right up and then stumble for a second once it comes back to idle but wont stall. Not sure if those issues are related somehow.

Any Ideas,
Matt
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Old 05-29-2019, 10:31 AM
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Clean the throttle body.
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Old 05-29-2019, 11:46 AM
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Originally Posted by carfixer007 View Post
Clean the throttle body.
Ok, I will do that today with some CRC and a toothbrush and report back!
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Old 05-29-2019, 07:38 PM
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If that doesn't work, next I'd be looking for a vacuum leak. Vacuum (and therefore a leak) is highest with throttle plates closed and a leak is least-easy to deal with at idle.
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Old 05-30-2019, 12:20 AM
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So I have some good news, I cleaned out the throttle body with some "Deep Creep" spray and made it spotless. When I first started the car and snapped the throttle it still stumbled coming back to idle. I'm guessing that was because of the leftover cleaner in the intake manifold. I drove the car hard down the road one time and it seemed to have solved my second problem. The car still has a slight misfire at idle, but no longer stumbles after snapping the throttle.

I noticed the car leaks a small amount of oil at the gasket where the intake manifold bolts to the motor. Could that contribute to the misfire? I tried spraying carb cleaner on the leak with no change in idle.

Matt
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Old 05-30-2019, 01:05 PM
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Originally Posted by pjmia96 View Post
So I have some good news, I cleaned out the throttle body with some "Deep Creep" spray and made it spotless. When I first started the car and snapped the throttle it still stumbled coming back to idle. I'm guessing that was because of the leftover cleaner in the intake manifold. I drove the car hard down the road one time and it seemed to have solved my second problem. The car still has a slight misfire at idle, but no longer stumbles after snapping the throttle.

I noticed the car leaks a small amount of oil at the gasket where the intake manifold bolts to the motor. Could that contribute to the misfire? I tried spraying carb cleaner on the leak with no change in idle.

Matt
Those intakes will leak internally and cause a misfire.
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Old 05-30-2019, 01:30 PM
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Originally Posted by carfixer007 View Post
Those intakes will leak internally and cause a misfire.
Yup, what carfixer007 said ^^^^^^ .

. . . and are you sure that oil is coming from the intake? It might be from a rocker cover gasket and oozing it'* way to the intake gasket.
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Old 05-30-2019, 01:30 PM
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Originally Posted by carfixer007 View Post
Those intakes will leak internally and cause a misfire.
Do you know an easy way to test and see if the intake manifold has an internal leak, could I put a vacuum gauge on it? I Sea Foamed the car today and smoke only came out of the exhaust and nowhere else.
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Old 05-30-2019, 01:32 PM
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Originally Posted by CathedralCub View Post
Yup, what carfixer007 said ^^^^^^ .

. . . and are you sure that oil is coming from the intake? It might be from a rocker cover gasket and oozing it'* way to the intake gasket.
I did replace both of the rocker cover gaskets, but I will double check and make sure that oil is not leaking out of them.
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Old 06-03-2019, 02:06 AM
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So, the valve cover gaskets are not leaking, and the intake manifold has a very slow oil leak at the corners of the manifold, I hear that'* a GM thing! I replaced my timing chain and sprockets as well as the magnet back in February. I hear a small clicking sound by the harmonic balancer, the rubber does not appear to be rotted but I am wondering if I didn't perfectly center the crank sensor and it is being bumped by the relucter ring throwing off ignition timing?

We will see!
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