rattling noise/engine power - dropoff

[cyberhun]

My 86 Bonneville engine (V8 5L with lots of miles on it) sometimes makes a rattling noise but only when accelerating at a very specific rate. If I step on the gas just a little harder, the noise stops, or if I ease off on the gas just a hair it also stops. When the noise is occurring, engine power drops. It doesn't happen when idling, and it never happens during de-acceleration, just when accelerating at maybe 80-90 percent of gas-pedal-depression. How warm the engine is doesn't make much difference. The sound comes from the engine, not anywhere else on the vehicle. It reminds me of the noise a cold diesel engine makes when you start it on a cold day, but it's not 'dieseling', in the sense that when you turn the engine off it keeps running --- that doesn't happen.

-Can I fix it by getting a timing light and fine-tuning the timing? Or would fixing this probably require major open 'engine-case' surgery and replacing lifting rods or cam-shafts or other hardware?

[hotrod_chevyz]

Sounds like the timing is off. This is usually an indication that the timing chain has some wear.

305's usually do that under a load, unless the timing is perfect. And some do that even when the timing is dead on.

You might want to check the timing chain just to make sure its not about to crap out on you.

[cyberhun]

I'm a bit of a newbie when it comes to this --- I looked at the procedure for accessing the timing-chain on autozone.com, seems like you need a whole bunch of specialized tools I don't have like a 'balancer installer', 'woodruff key' and stuff. Not sure how I'd check the timing-chain to see if it's about to crap out anyway, it works, so it's not completely broken, that's all I know other than there is this rattling noise and loss of power sometimes. Maybe be better if I didn't bite off more than I can chew, so to speak.

-Do you have to remove the front cover to simply adjust the timing? Can't you just hook up the timing-light somehow and watch where some notch on a rotating gear/wheel somewhere lines up with stationary marks by using the freeze-motion effect of the timing-light, without removing all the parts that need removing to get the front cover off?

-What else besides a timing-light do you need to adjust the timing? Do you need specialized tools?

-When I try to get in depth info about "Timing Specifications" on Autozone.com, it seems you have pay money --- is there a way to get this info online for free? I only have the owners manual.

[hotrod_chevyz]

To adjust the timing you loosen a bolt at the base of the distributor, this allows you to turn the distributor clock or counter clock to adjust the timing. All you need to do this is a distributor wrench, and a timing light. It should say what the timing is supposed to be set at on a sticker under the hood. If you dont have that just let me know and i will figure out where its supposed to be. Once the timing is set tighten the bolt at the bottom of the distibutor and check the timing again cause as you tighten the bolt back up they sometimes move.

To check the timing chain is pretty simple.

1: take off the distributor cap.


2: using a socket, ratchet and extension you can turn the crankshaft back and fourth by using the center bolt on the balancer pulley. Start by turning it counter-clockwise. turn it around about 1/4 turn or so, or turn it till the rotor in the distributor if facing the front of the car so you can see it.

3: mark where the rotor inside the distributor is pointed. a magic marker is useful for that. Mark both the rotor, and the base of the dist, and make sure the marks line up perfect.

4: next, click the ratchet so you can send the motor back the other way, clockwise. Barely rotate the engine clockwise with the ratchet maybe 5 degrees and then look to see if the rotor inside the distributor has moved.

This is a way of testing how much "slop" is in the timing chain. If you have rotated the motor more than about 5-10 degrees and the rotor hasnt moved from the mark any its time for a timing chain.

If you cant find the timing marks somebody may have removed the tab, and it may have to be set by ear.

[maxwedge]

These symptoms could be a result of an inop egr system, first, second if the chain were worn, the timing would be retarded and the chance of " pinging' would be less.

[cyberhun]

Originally Posted by maxwedge

These symptoms could be a result of an inop egr system, first, second if the chain were worn, the timing would be retarded and the chance of " pinging' would be less.

Are you sure 1986 Pontiac Bonnevilles have 'exhaust-gas-recirculation' systems? I thought they were for later model vehicles.

[maxwedge]

Egr has been mandated from approximately 1975, Check this link and lets been done with this! http://autozone.com/servlet/UiBroker...3d800d462d.jsp

[cyberhun]

Originally Posted by maxwedge

Egr has been mandated from approximately 1975, Check this link and lets been done with this! http://autozone.com/servlet/UiBroker...3d800d462d.jsp

You're right --- like I said, I'm a newbie when it comes to automotive technical nitty-gritty.



Thanks for the response.

[JustSayGo]

Take a close look at the distributor. The timing may be to far advanced at lower RPM if the distributor shaft is sticking and the advance springs can't retard timing as RPM decreases.

[hotrod_chevyz]

Those are electric timing advance they dont have springs or vac advance. My sister use to have that same car, finding a sweet spot in the timing without any spark knock is a total pain in the neck. This is true for most 305's with high miles and the computer/timing system the car in question has. Usually the only way to get it to stop clattering is with a fresh timing chain, and a older style HEI wich does have a recurvable vac advance style HEI dist.

I say all this because my sisters car wich is the same as we are talking about was acting the same way, and was also high miles. I didnt figure it out for a long time. I put magnaflow exhaust, timed it several times, even had the computer replaced, none of wich helped. I put a older style distributor in it, a new timing chain, and la-de-da it stopped clattering and had more power than it ever did.

And let us all not forget those have a throttle position sensor built into the carb. If those have a "dead spot" it will not run right, and this could explain why if you let off the gas or increase the gas some, the noise stops.. Those carbs are so expensive to get rebuilt RIGHT that its a lot easier to get a older HEI with vaccume advance, and forget about the computer. And doing that will not affect the emissions system, as it is has a carb. it will however run more efficiently with good spark timing. The only downfall to that is the check engine light would stay on.

[Bassasasin]

Is it a clatter or a rattle? Lifters or detonation? Wrist pins or lifters?

Just some thoughts.

Keep us posted. Im having similar symptoms.

Good Luck

[cyberhun]

-I don't know how to tell a wrist pin noise from a lifter noise --- sorry.

-Detonation, like in premature detonation? I don't think so, but again, I can't be certain.

-Lifters? Maybe.

-It sounds like a metallic, rattly noise --- very similar to the noise that a badly tuned diesel engine makes when it accelerates.

-If I drive like an old lady, I never hear the rattly noise at all. It only happens at highway speeds, or when accelerating at 78 to 83 percent gas pedal-depression --- if the noise starts and I punch it, the noise stops, and it also stops if I let up on the gas-pedal even slightly.

-I don't know if this is related, but when the engine has just started to warm up, shortly after being started, if I punch the gas-pedal hard --- it'll completely stall, but if I very carefully and slowly depress the gas-pedal it'll accelerate without stalling.

-It might take a couple/three days until I can get the time and tools all lined up to try Hotrod_Chevy's "Determination of Timing-chain Play" procedure, but I'll post the results here as soon as I can.

Happy New Year to all ...

[hotrod_chevyz]

You can go ahead and see if any hoses are cracked, and maybe see if the EGR is stuck or not working right. maxwedge could very well be right its hard to say by guessing when it comes to a high mile motor, so your probably best to check all you can starting with the simple stuff first.

Most 305's with carbs are noisy like that. its kinda their thing.

Its also hard to say if the stalling is related or not. It could be the choke/carb, or this could also further indicate problems with the EGR system.

Good luck and happy new years to you also. Keep us posted on your findings.