Engine braking effect when throttle is closed

[AlmostStock]

On my 2005 Blazer 4.3 V-6 with 5 speed manual transmission it takes several seconds for the engine to return to the normal idle after the throttle is released while driving. This limits the engine braking effect somewhat and I find it annoying.

Because I have noticed this on many other computer controlled vehicles I believe it is a normal trait of some on board management systems and technically not a problem. My question is why do they do this and is there any way to make these FI pcm controlled engines deliver instant engine braking on throttle release like the old carburetors did? Thanks.

[curtis73]

I don't think there is any good way of accomplishing what you want. As a owner of three of these 4.3s (two blazers and one truck) I've noticed they are particularly bad about it. They seem slow to respond to both accelleration and decelleration. GM spends weeks on each engine family just on drivability so it makes me wonder why the 4.3 and the 2.8/3.1/3.4 families are so lacking in this department. For the 4.3 it seems to be an injector pulsewidth thing. For some reason GM decided to do multiple pulses on the injectors for acceleration enrichment instead of longer pulsewidth. They probably chose it for fuel pressure reasons, but I think its a choice that leaves 4.3 owners a little vexed. It sure ticked me off. Not sure why the engine braking doesn't come in right away but it may be for a similar reason.

You could try relocating the MAF closer to the throttle body but that usually only helps a tiny bit if at all. Sometime try taking your foot almost off the pedal but not quite the whole way. The problem might be in the IAC. Going all the way back to idle position would let the IAC compensate as the computer sees fit. If keeping the throttle cracked lets it return to nearly full braking, then it might be something that can be programmed out of the IAC function.

By the way... are you sure about the engine and year combo?

[MagicRat]

Originally Posted by curtis73

By the way... are you sure about the engine and year combo?

Yup, he's right. GM sold this combo for list about $1000 less than the automatic trans, around here. For what you got, the price, especially for the 5 spd was an amazing deal. I seriously considered buying just this exact combo, this past spring.

This drivability problem is engineered in to reduce emissions, (NOX, I believe, but I am not sure). Many GM carbureted engines from the early/mid 80's had a mechanical valve which did the same thing.
Essentially the valve behaved like a temporary vacuum leak, and was very irritating. The valves could be plugged. Maybe these engines have a valve that can be plugged, too, (I am not sure) If it is ECM related, perhaps an aftermarket PROM performance chip might help??

[sracing]

Originally Posted by AlmostStock

My question is why do they do this and is there any way to make these FI pcm controlled engines deliver instant engine braking on throttle release like the old carburetors did? Thanks.

Yes, there are a couple params in the ECM software that control this and can be changed. You however need to purchase the software/hardware to do this, or find a shop that can. The IAC reaction speed and throttle shutdown can be modified close faster.

Jim
SR Racing

[jveik]

yeah ive noticed on my dads 6.0 liter truck it wont even unlock the torque converter when you let off the gas for over 10 seconds. my old 73 chevy truck with a 350 is awesome at engine braking since it has a holley carb. the problem with mine is that 2 or 3 cylinders have almost zero compression so i have to buy a new engine

[AlmostStock]

Thank you all for taking the time to respond. I also have a 2001 4.3 Blazer (auto) that exhibits this same tendency. As a test, if I release the throttle at speed and then put it in neutral, (or disengaged the clutch) the engine will only drop to 850 rpms, then drops down to the normal 650 after the vehicle comes to a stop. This is why full engine braking is not achieved. This is obviously controlled by the pcm for one of the reasons mentioned above, possibly emissions.

I have zero experience changing chips or reprogramming ecm's / pcm's so I need to study up on this. My 2001 GM service manual calls it a PCM (powertrain control module) so I assume when you guys are saying ecm (engine control module?) you are referring to the same thing.

Like I said earlier this is technically not a "problem" as in a defect or part failure, and most drivers who go from gas to brake would not even notice it. But sensitive drivers who like to coast down before braking appreciate a system that drops rpm's to minimum as soon as the throttle is released. If I am able to achieve this I will be surprised but I will let you know.

BTW- Yes Curtis it is an '05. Chevy had a short production run for the 4.3 Blazers (2 door only) that ended late '04 or early '05. Mine was made in December '04. I'm sure that there are still a few left on dealer lots but most are automatics.

[curtis73]

Ah. Understood on the year. I was off by a year or two. Anything made after 1978 is a mystery to me