Whole lotta shakin going on

Hey folks, I'm back.

My 69 Mustang Grande is back from the transmission shop and shifting smooth...

New problem... :roll:

When in gear, with the foot on the brake, the engine shakes hard. Steering wheel bounces about an inch!

A friend who is a racing engine expert said to check the timing. He said it sounded like it wasn't running on all cylinders.

I tuned it with a vacuum gauge, checked the spark plugs and cables, and the shaking persists.

The Vacuum needle was also shakey and maxed out around 16. It steadied with higher RPMs. According to the Tech Archive, this could be a problem with the valve guides.

What do you think? Is there a trip to the machine shop in my future?

I would check the timing. Also, are the brakes power assist? Maybe vacuum leak somewhere. Also are all cylinders firing? Does it seem fine in neutral? With the brake applied?

The brakes are power assist. I'll check the pump and lines, perhaps something got knocked around while they were working on the tranny.

Smooth as can be in neutral with the brakes on though.

What camshaft do you have in your engine??

auto transmis, bro?
("OK in neutral")
a lill more in sig would help diagnose.
(don't B membah ur last thread, sorry)

yes Automatic Transmission. To the best of my knowledge the camshaft is stock for a '69 250.

It only shakes when I brake in Drive, 1st or 2nd. Park and Neutral are smooth. I checked out the power brake lines, and they are loose at the intake manifold. I'll tighten those down tomorrow and see if it makes a difference.

I am guessing that you have an automatic transmission since you mention one foot on the brake and the smooth shifting. All of the idle concerns started immediately after the transmission repair correct? Did the transmission shop do the install or did you just have them bench build the unit and you installed it yourself? You mentioned that the idle is smooth in neutral, how much faster is the engine speed between drive and neutral or in gear and neutral? Is the idle speed faster now than before the repair? When you apply the brakes how does the pedal feel? Is it hard or soft? Is it different than before the transmission repair? Are there any new noises you hear when applying the brakes maybe a "swishing" sound?

I cant imagine your valve guides just suddenly failing and causing your concerns. It's easy to get tunnel vision on these things and freak out and go to the worst cause first.

Start with the simple stuff and eliminate it one by one. If it were mine I would check for a vacuum leak first paying special attention to the brake booster and check valve and hoses. Then check the cap and rotor for signs of arcing. Then check the plug wires for continuity while checking each spark plug to make sure it is not fouled or crudded up. Eliminate all of that first before you even think about an internal engine issue.

You mentioned that you set the timing with a vacuum gauge instead of a timing light. Is this because you don't have access to one? I think that some of the big box parts stores will lend them out if you don't have one. I think it may help you with your diagnosis process.

Sorry about all of the questions. I know first hand how disappointing it can be when you get a major repair done and something else comes up, hopefully it will be something small and easily addressed. My bet is that it is.

Good Luck,

Kevin

Thanks Kevin,

Yes, Automatic tranny. the shop rebuilt and reinstalled. they also ended up replacing my radiator and making new coolant lines. It is highly likely they had to disconnect the brake booster lines in the process. The shaking is suddenly new since the repair, so, yes, it seems unlikely that it is caused by wear.

Tuning: I do have a timing light and I used it in conjunction with the vacuum gauge today. I have the electronic timing system (no points) and a 40,000w coil with hotter plugs, so the timing notch isn't necessarily right anyway as the whole thing can be timed ahead, but I like to check with both. What I don't have is a Tach, but I think I will pick one up tomorrow so there will be less guess work on the idle speed.

Engine speed is definitely higher in neutral than in gear. If it's a timing issue, it is certainly possible that the "shake" is just not noticeable at the higher engine speed.

I noticed that the brakes were softer when I picked up the car, but after 4 weeks, I thought maybe I just hadn't noticed before you know?

The plugs looked good. I will check the cables again. I will check the cap and rotor, good point, though, with the electronic system there is very little that can go wrong there. Still...

As I said above, I can wiggle the brass adapter connecting the booster tube to the intake manifold. It definitely needs tightening. Let's hope that's it.

In case you are wondering...

I bought a tach and got this thing tuned much better. My idle speeds were way too high. RPMs at idle are now 650 in drive and 1100 in park. Most of the shaking went away and it's more like it was before the tranny repair.

The brake vacuum line was still clearly a problem the brass adapter that threads into the intake manifold was able to be tightened more than three full turns.

Sadly I had to cut a metal vacuum tube to get it where it could be turned. I'll replace the tube tomorrow and get back on the road!

I'm glad it was what you thought and it sounds like you are about to be on the road again!

shoot 4 an even lower idle if possible.
I think the "CI archive" listed on each Index page has the spec.

Conclusion of the matter...maybe.

Replaced the brake booster tubes. Less mushy brakes now, but the shaking came back.

a teenage kid working at OReillys took one listen to it and said it was the valve body on the transmission. He recommended loosening the the throttle linkage. Bingo. No shake whatsoever.

Feels like I'm just treating the symptom though.