Stumble while decelerating
Thread Starter
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
Stumble while decelerating
I've noticed lately that my 87 has a bad stumble when decelerating at low RPM's. For example, if I start to slow down heading towards a traffic light or stop sign, I take my foot off the gas and let the engine slow the car. Once I get to around 1100-1200 RPM, there is a sudden loss of engine braking as though the throttle plate was forcibly closed to the point whre it feels like the engine stops.
It doesn't stop, but it's more like a immediate drop from that 1200 RPM to about 400 RPM. The engine doesn't run rough, but just this big bog or stumble.
Ideas?
It doesn't stop, but it's more like a immediate drop from that 1200 RPM to about 400 RPM. The engine doesn't run rough, but just this big bog or stumble.
Ideas?
Safety Car
Joined: Oct 2007
Posts: 4,425
Likes: 229
From: Pennsylvania
I have an 85 4+3 with the same condition. It never stalls but it does help if I don't wait so long to push the clutch in when coming to a stop. I'll be glad to hear any suggestive hints also.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
bog
sounds like the EGR valve is sticking open.
Wait till the car is cold, see if you can feel underneath the manifold plenum where the egr valve is, and see if you can move the diaphragm with your finger tips.
You have to use both hands, get close, with your legs on each side of the suspension control arms and kinda sit on the tire. If it moves easy, it's probably all right. If it's sticky or ratchety it might just need cleaning.
You might have to move the plenum out of the way to get a better feel for it.
1 hour to remove and replace the plenum if the gaskets let go easy. When installing a new gasket, spray the gasket with a graphite spray, or Pam cooking spray or a copper based gasket spray
Wait till the car is cold, see if you can feel underneath the manifold plenum where the egr valve is, and see if you can move the diaphragm with your finger tips.
You have to use both hands, get close, with your legs on each side of the suspension control arms and kinda sit on the tire. If it moves easy, it's probably all right. If it's sticky or ratchety it might just need cleaning.
You might have to move the plenum out of the way to get a better feel for it.
1 hour to remove and replace the plenum if the gaskets let go easy. When installing a new gasket, spray the gasket with a graphite spray, or Pam cooking spray or a copper based gasket spray
Supporting Vendor
Joined: Mar 2008
Posts: 1,762
Likes: 14
From: Cumming Ga
I've noticed lately that my 87 has a bad stumble when decelerating at low RPM's. For example, if I start to slow down heading towards a traffic light or stop sign, I take my foot off the gas and let the engine slow the car. Once I get to around 1100-1200 RPM, there is a sudden loss of engine braking as though the throttle plate was forcibly closed to the point whre it feels like the engine stops.
It doesn't stop, but it's more like a immediate drop from that 1200 RPM to about 400 RPM. The engine doesn't run rough, but just this big bog or stumble.
Ideas?
It doesn't stop, but it's more like a immediate drop from that 1200 RPM to about 400 RPM. The engine doesn't run rough, but just this big bog or stumble.
Ideas?
Thread Starter
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
Thanks!!
Would a bad IAC also cause a cold idle surging? When I start the car when it's cold, the idle will be fine for the first 20 seconds or so and then the idle RPM's begin to surge. The idle speeds will change from 700 RPM to 900 RPM and back for about a minute before the idle smooths out and the engine warms up. This only happens when cold.
Would a bad IAC also cause a cold idle surging? When I start the car when it's cold, the idle will be fine for the first 20 seconds or so and then the idle RPM's begin to surge. The idle speeds will change from 700 RPM to 900 RPM and back for about a minute before the idle smooths out and the engine warms up. This only happens when cold.
Supporting Vendor
Joined: Mar 2008
Posts: 1,762
Likes: 14
From: Cumming Ga
Thanks!!
Would a bad IAC also cause a cold idle surging? When I start the car when it's cold, the idle will be fine for the first 20 seconds or so and then the idle RPM's begin to surge. The idle speeds will change from 700 RPM to 900 RPM and back for about a minute before the idle smooths out and the engine warms up. This only happens when cold.
Would a bad IAC also cause a cold idle surging? When I start the car when it's cold, the idle will be fine for the first 20 seconds or so and then the idle RPM's begin to surge. The idle speeds will change from 700 RPM to 900 RPM and back for about a minute before the idle smooths out and the engine warms up. This only happens when cold.
Safety Car
Joined: Oct 2007
Posts: 3,816
Likes: 0
From: :45 minutes from everywhere / E-I-E-I-O
Check that magazine article on TPI Tunin'... There was a reference to acceleration enrichment, AND, 'DEceleration enLEANMENT'. I don't remember the process exactly, but maybe it's gone awry on his car (injector problem???)?
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