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88 Sub Cruise Control Fading

I've got an 88 Suburban 350 with cruise control. I drove from NC to FL and back a few weeks ago. The crusie control was acting uup most of the way.

I'd bring the truck up to 70mph or so and set the cruise. It would hold for a few minutes, then it would start surging ever so slightly. After maybe 30sec of the light surge, the truck speed surge would stop, then the speed would begin dslowly dropping. If I turned off the curise, drove it back to 70mph, the same thing would happen. It didn't matter if the AC was on or off.When it would start to slow, I could give it a little gas and bring it back up and it would hold it for another minute. When I gave it gas I could tell that the servo was pulling on the throttle because it didn't take much force to depress the pedal.

I(t felt like the servo was starving for vacuum.

i took my mityvac and checked the brake vacuum release, it's holding. The AC circuit has a slow leak, not much. The vacuum reservoir holds fine.

What kind of vacuum should the engine be pulling? I'm getting 21" at idle in heutral, 19" in drive. At 70mph flat highway I get 15". Up a modest grade at 45mph, about 1/8 throttle, it drops to less than 10". Mild acceleration 1/4 to 1/2 throttle it's less than 5" and WFO it's

1-0".

Around town the cruise works fine. I'm wondering if on the trip the catalytic convertor is heating up and blocking off the exhaust? Would that cause the manifold pressure to degrade and screw up the cruise control? Or is it more likely the servo?

I don't have the time right now to drive around for a few hours to get the porblem to repeat itself and it would nice if I could fix it _before_ my next trip. :-)

-RC

R.Clarke spam snipped-for-privacy@BlocKmindspring.com RTP, NC, USA

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R Clarke
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91k miles and it runs fine. Probably not a plugged cat, just wondering if engine vacuum is normal.

-RC

R.Clarke spam snipped-for-privacy@BlocKmindspring.com RTP, NC, USA

Reply to
R Clarke

You could try spraying contact cleaner/lubricant in the turn signal cruise control stalk to see if corrosion is causing intermittent operation. At the very least, this should improve the operation of your windshield wiper switch.

You don't mention if you have an automatic or manual transmission. Check the operation of the cruise control shutoff switch located above the brake pedal and also the one above the clutch on a manual transmission. A malfunctioning shutoff switch could cause intermittent operation.

As far as all of that vacuum stuff you mentioned is concerned, I haven't a clue.

Reply to
One-Shot Scot

Not a bad idea, except I can tell it's still "on". The servo is actuated, it just isn't pulling on the throttle hard enough.

Automatic.

I drove for 30 miles today trying to get it to malfunction again, while I had a vacuum gauge tee'd in at the servo. It worked perfect the whole time. It acted up for three days over 1000 miles two weeks ago. :-/

I guess I'll have to take the vacuum gauge along on my next trip in October.

-RC

R.Clarke spam snipped-for-privacy@BlocKmindspring.com RTP, NC, USA

Reply to
R Clarke

RC,

The vacuum you've described is bang on the money for a healthy 350. Isn't the cruise control module on that thing electric anyways (it is on my 88' C1500)?

Doc

Reply to
"Doc"

Knowing the vacuum readings are normal is a big help. All I've ever done in the past were idle readings.

There's an electric control module under the dash. It runs the vacuum powered servo that is bolted on the left rear corner of the intake manifold. The vacuum circuit goes something like this...

Vacuum is tapped off the intake manifold where it goes to a check valve. The check valve has two branches-

A) One goes to the cruise control servo. The servo has a second vacuum line that goes to a release valve on the brake pedal.

B) The other branch of the check valve goes to a tee where it goes to a) the vacuum reservoir. b) the AC/heater controller

I've replaced all the small rubber lines. I haven't messed with the hard plastic line that runs through the firewall to the AC, nor the rubber line that runs to the brake pedal. The brake pedal line tests fine. There's a slow leak in the AC vac line, and if it's anything like my 85 Caprice it's just a dry o-ring in the AC control valve. All the vent doors operate normally.

I can disconnect the AC circuit and block it off once the weather cools down. I'm taking a trip in October so I can test it then.

The most frustrating part is it doesn't act up for 20-30 miles until the underhood temps get good and hot. I may have to take a 100 mile trip just to figure this thing out. My 85 Caprice was easy to figure out when it started acting up. The cruise control would shut off every time I turned on the rt. blinker. I wiggled the thin black wire bundle coming out of the stalk and now it's working fine again. :-)

-RC

R.Clarke spam snipped-for-privacy@BlocKmindspring.com RTP, NC, USA

Reply to
R Clarke

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