MALIBU Vacuum/Defrost

[Ribbedroof]

The original post was written at lunch on a damn iphone (which I hate) so typing and editing are not the easiest/best.

Typically, when the system goes to defrost upon acceleration, there is a vacuum leak in the system. I would reference the attached diagrams and start at the connection from the engine to the vacuum ball, and follow it from there. As the lines are hard plastic, it is common for them to break or develop cracks over the 35+ years they've been around.

By design, the system is designed to default to defrost in the absence of a vacuum supply. This is done for safe operation in colder climates. Should the system lose vacuum, the air will still flow to the defroster.

 

[78Delta88]

I've been doing some legal stuff at moment, so been going back and forth, but was question I was going to ask to rule out maybe the plastic line broke, and if in fact you had plastic or the rubber line.

Sometimes on a 30 plus year old car the easiest is to just yank off and replace the hoses and lines. Along with that check all wires and connectors.

 

[78Delta88]

......> By design, the system is designed to default to defrost in the absence of a vacuum supply. This is done for safe operation in colder climates. Should the system lose vacuum, the air will still flow to the defroster???

Yes. With out vacuum there is no signal to the actuator to move the door to the vent or floor position. Door stays in position to route warm air to the dash outlets to defog/defrost the windshield.

 

[78Delta88]

If you lack vacuum and then it comes on during acceleration, then suspect somehow ported/timed vacuum signal is getting across to where it shouldn't be. Odd yes, but you have no idea what was done by the previous owner(s) or mechanics, prior to you getting the car.

 

[78Delta88]

LINK>>>

Do it Yourself Automobile Repair Information - Mitchell 1 DIY | eAutoRepair.net

 

[Ribbedroof]

I have access to Alldata, I find it's value on older stuff to be pretty limited. And brand-new as well. I feel like it;s good for vehicles that are a couple years old back to the late 80s.

Almost nothing for my 81s
Perhaps Mitchell is better?

Hard to beat the OE books, though, for thoroughness IMO

 

[78Delta88]

Not sure, but thinking AllData (tm) came out about '99 - 2000-ish. Haven't used it since about 2001. Had good luck so far with the Mitchell.

 

[Ribbedroof]

Just looked....alldata was founded 1986. Makes sense on coverage, then

Free from work account, so......

 

[Dezimade]

You need to find out why such a small leak is causing a stubble on acceleration.

You have a bigger leak somewhere that is throwing off your air/fuel mix. Leak is causing a lean issue.

I see. I will definitely get on it as it’s a bit annoying. Dez.

Forget the Haynes and Chilton manuals. You want the GM factory shop manual. It will include MUCH MUCH more useful information about pretty much every subsystem on the car. This is the GM manual for 1988 Monte Carlo/Full size.

View attachment 236667

A Chilton or haynes manual is less than half as thick, and covers every Chevelle year ever made. Contains only the most basic information. You WANT the GM manual. I paid $14 for this one on ebay a month ago. It can be bought new on CD for more...I prefer having the actual paper book, but some people like being able to print the pages they need for a given task.

great advice. Thanks man

 

[1320chevy]

gET A VACUUM GAUGE AND MEASURE HOW MUCH VACUUM THERE IS IN THE MANIFOLD. tHEN CHECK VARIOUS VACUUM LINES AND SEE IF THE READING IS THE SAME. IF NOT, THERE IS A LEAK BETWEEN THE SOURCE AND YOUR GAUGE. FIND AND REPAIR IT. THE VACUUM RESERVOIR CAN CRACK AND LEAS TOO. your ENGINE Vacuum should be at least 16 inches. You can sometimes get away with a little less bun not a lot. Aside from physical leaks, a fault in the engine like a bad valve, CAMSHAFT OR LIFTER OR, TIMING THAT IS WAY OFF FROM WHERE IT BELONGS.