Engine & compression braking

[Steve Williams]

Greetings, My 84'/1.9 @ 104 K/miles runs pretty good going up these Colorado grades but when coming back dowm and using the engine to augment my brakes when I begin to accelerate runs a touch rough until I give it more gas & rpms. Have run basic Bentley (home mechanic) diagnostics on sensors-stabilzers,..... (ohm/voltage). I may have thermostat issue (see new post in cooling section) as engine does not maintain temperature on descent. Could this be a possible injector issue ?
Thanks, Steve

[Capt. Mike]

Not likely an injector issue. During decel, the engine reaches maximum vacuum which affects distributor timing. You may not be getting correct timing during initial change-over back to acceleration. Check your distributor advance mechanism and timing.

There is a throttle valve/decel/idle switch that comes into play (Bentley 24.33) and a full throttle enrichment switch (Bentley 24.34)

It's not a thermostat issue.

Before I'd got too deep into FI issues, which fuel are you running? Colorado is notorious for cutting octane below what VW requires. You should be running 87 R+M/2 method (Federal pump label). I've noticed much of Colorado's regular is sub-octane, as low as 83. Run 87 regardless of what the pump names it. There is a gas and octane thread on the TIPS forum. Wrong octane can create a good stumble on changes in acceleration.

[ronwolffjr]

I hope this helps with your engine braking problem.

As Capt. Mike said, there is a deceleration valve (I believe yours is electronic). Check it carefully, along with the distributor advance.

It sounds like your cylinders are being flooded slightly, and when you accelerate the extra fuel burns off as if it is flooded. I would check that out ASAP, as many an engine has had gone to it's eternal rest due to the fact that there is more fuel in the combustion chamber than sufficient oil viscosity.

You mentioned the thermostat -- is the engine running hot down these grades? If it is, most likely you are injecting too much fuel - and the lack of lubrication (due to the fuel breaking down the oil slightly, will cause friction and give you a high-temp reading. Also, check your oil level -- down a grade it tends to gather near cyl. # 1 and #3.

Plus, check the following:

- Vacuum lines, often the culprit, especially if you are running hot - they crack.

- Depending on where you are in CO, make sure that your ignition is timed for your altitude. (don't depend on auto-altitude correction, it blows).

- Learn everything you didn't want to know plus more about your deceleration valve. That is your engine braking buddy, get intimate with it - start with a drink, maybe dinner first. This needs to be on top of it's game.

- Capt. Mike has a great point regarding fuel - make sure that you have some punch in the octance arena, you'll need it to burn off the extra "down the hill to grandma's house" fuel.

THE TEST: When you are approaching the end of a long downhill run, but are not needing to brake yet (give yourself about 5-8 seconds of coasting) - press in your clutch, if auto, put it in Neutral. If the engine stalls or the RPMs drop to near stalling, back to drawing board.

If there is little to no change in RPM's, congratulations you probably fixed it.

I am fairly knowledgeable about Fuel Inj. - but not oil. I don't want to give you the wrong advice here so someone may want to chime in -

If this VW is a hill-monster, up and down, I do know as a fact that more fuel than normal goes into the combustion chamber.

What I'm not too sure about is putting thicker-than-normal oil in the crankcase, to reduce wear as a result of fuel richness. I don't want to say "do it" as I like to make sure that I know what I'm talking about prior to giving any assistance. So, either ask around, or maybe the Capt. will come to the rescue. He got me out of some pretty sticky situations!

Good Luck

Heatster.

[Capt. Mike]

All good advice. I'll only add in NOT to go to a thicker oil as that would only be treating the symptom, not the problem.

One other area you might investigate is valve stem seals. As mentioned, the engine in decel going down hill pull maximum vacuum. If you are pulling oil in through worn valve stem seals, you would get rough running for a minute or two until it clears out. Is there any exhaust smoke then? Another indicator of valve stem seals, do you have some smoke on start-up that quits after a few miles when the vehicle has sat for a couple weeks? Further info under the ENGINE forum, Cylinder head topic.