1986 Corolla hesitates when coasting in gear

[LurkinHomer]

Hi There,

I was hoping to find someone here who knows something about the Toyota 4A-LC engine and feedback carburator system...

I've got a 1986 Chevy Nova (rebadged Toyota Corolla) and have been attempting to trouble shoot what seems to be a hesitation probelm under high vacuum throughout most of the three years that I've had it. The car runs, but it "chuggles" while coasting in gear (manual transmission), which is increadibly annoying!

I've checked and/or replaced the following:

1. The carb Auxilary Acceleration Pump diaphraghm had a hole in it allowing fuel into the vacuum hoses. I replaced the diaphragm and cleared all of the affected vacuum hoses of fuel.

2. I verified that all of the vacuum hoses were routed correctly
according to the map on the underside of the hood. All checked OK.

3. Followed the emmision control system test procedure in the Haynes manual. All checks OK.

4. Replaced the cylinder head with an overhauled one (together with a full set of new gaskets) after finding a burnt valve on the #1 cylinder (I figured I may as well, I had it apart anyway).

5. Replaced and rigged the throttle position switch (the car has the California emmisions package) according to pages photocopied from the Toyota service manual.

Most recently (within the past two weeks), I've replaced the following (not the first time some of these were replaced, obviously):

6. Distributor, including cap, rotor and spark advance system

7. Spark plugs

8. PCV valve

9. Fuel filter

10. Air filter

I've also double checked the timing and fiddled with the idle speed repeatedly. Do you think any of this would have solved the problem? Nope! The car still does what it has always done (to varying degrees of severity), no matter how many "hard faults" I find. It is almost as if it is mocking me... Could the TVSV be sticking? The problem seems to get worse when as the engine warms up (although it isn't overheating). What about the electronics box that controls the various vacuum valves? (Although this too seems unlikely, because the problem happens with the throttle closed, and the throttle position switch replaced in item 5 above turns the computer off when the throttle is retarded to idle.)

Any ideas from the experts and /or other Corolla owners out there?

Thanks very much in advance for your help!

Kevin

[Dicipleofthelie]

by chugging.......do you mean rough idle.........or.......is it more like.....extremely low rpms?

[LurkinHomer]

Hi There,

>by chugging.......do you mean rough idle.........or.......is it more
>like.....extremely low rpms?

No, what happens is that the whole car shudders when coasting in gear. (ie no pressure on the gas pedal while coasting in gear up to a light.)

I figured at first that it might be a valve problem, so I did a compression test. That is how I found the burnt valve in the number 1 cylinder. While installing a rebuilt cylinder head, I discovered fuel in one of the vacuum lines coming off the carb, and I traced the source back to the auxiliary accelerator pump. While doing that, I also discovered that the idle cutoff switch for the emmisions control system wasn't rigged properly and that the switch wasn't working, so I fixed that too.

You'd think all of that would have had some effect, but it didn't so I kept looking. I subsequently found that the diaphragm on the vacuum advance system on the distributor had a hole in it and that the distributor cap and rotor were pitted, so I replaced those this past week.

Admittedly, car does run a lot better now than when I first got it, and I'm still happy with it (I refer to it as "my old dog". It has it's personality quirks, yet I take care of it and it takes care of me...) I'd just like to figure out what the source of the shuddering is. (For the record, the shuddering stops as soon as I press the clutch or take it out of gear, and it only happens when there's no pressure on the gas.)

Kevin

[reym]

Does it run well when you put on some speed? Is there any problem when accelerating from 0?
Why don't you try the clutch? There must be something wrong with it, i suspect the pressure plate or the clutch lining, if not check if your brake is sticking.

[Chris Corolla S]

Moved to tech....