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A bad cam?

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Morris

Morris

Yoda
Offline
Problem: wildly fluctuating engine vaccum freaking out my ecu and causing poor performance.

Attempted solutions:

1) Thorougly checked for vaccum leaks using the "spray" method. Replaced manifold gasket. Disassembled and inspected carb/throttle body. (my efi system uses the ZS150 as a throttle body).

2) Replaced weak ignition system. Replaced platinum plugs with copper cores.

3) Removed and inspected head. Replaced two galled valves and guides. Checked valved clearances several times.

4) Compression check (front to back): 130; 130; 129; 140. This was pretty much the score before the problem arose. I believe #4 is getting a little extra oil resulting in a higher number.

5) Checked accuracy of MAP sensor.


So is it time to pull the cam? Could I have flatted my lobes with my sticking valves?

Another option is to insert a small fuel filter in the MAP sensor vaccum line. This will smooth out the vaccum signal for the ECU, but will not treat the source of the problem.

I am so frustrated, I am ready to drive this beast into the ground before I send it to the crusher.
 
Pull the plugs.
Put it in fourth gear with the valve cover removed.
Push teh car and observe the rocker arms.
They should all move up and down the same amount if the cam is good (for a better method, use a dial indicator, but I'd just "eyeball" it at first to see if they all look about even....it's unlikely that they'd all be equally bad).

You did a hot and cold compression test...right?
 
My latest compression test was a luke warm test.

The heat of the engine seems to have no effect on the wonkiness of may MAP.
 
What the heck is an ecu. emision control unit?

Does texas require that stuff?

What makes you think you have fluctuating engine vaccum? Have you had a gage on it?

Fluctuating engine vaccum when? At idle? All the time like up to 3000 RPM?

How does it run down the street? What happens? Like third gear at 2000 RPM, rough smooth, jumps around?
 
ECU stands for either Engine Control Unit or Electronic Control Unit. My car is fuel injected and I have on onboard computer (ECU) that runs the fuel injection and ignition system. The ECU calculates how much air is going into the pistons by measuring engine vacuum. I can plug a laptop into my ECU and watch and record my engine stats (including vacuum) while I drive around.

The engine vacuum jumps around at all speeds. It gets very erratic at higher rpm. The end result is that the car surges and wastes a lot of gas. ( I can also see/record my AFR in real time )
 
AH, quick question - have you confirmed the vacuum readings with a mechanical gauge?

It's entirely possible that the vacuum sensor or the inputs to the ECU are damaged. I'd want to be absolutely sure it's a mechanical problem, not electrical....
 
Me too.

Have you considered a good set of SUs and make it really sing. LOL

I expect that something is wrong with this electronic stuff, we all know you can't trust computers.

In fact if you still have your SU's you could think about reinstalling them and see if the prob goes away. I think it will.
 
Yes. I have checked the computer's reading against a mechanical guage. I also hooked a vacuum pump up to the MAP sensor. It seems to be functioning perfectly.
 
So who is the expert on this injection system? Sounds like they might have a job.
 
[ QUOTE ]
Me too.

Have you considered a good set of SUs and make it really sing. LOL

I expect that something is wrong with this electronic stuff, we all know you can't trust computers.

In fact if you still have your SU's you could think about reinstalling them and see if the prob goes away. I think it will.

[/ QUOTE ]

Great One!
Patrick
 
From a hole I drilled and tapped on the square part of the manifold where the carb attaches.
 
[ QUOTE ]
right by the intake ports?
rather than at one end...

[/ QUOTE ]

No, right by the carb.

I don't think the locating of the vaccum is a factor, as it ran great for several months with the vacuum line where it is now.
 
Have up disconnected and plugged up the vacuum advance to be sure that it is not leaking at the diaphram?
 
I don't have a vacuum advance. I will probe my throttle shaft with a stethoscope later tonight. I have thoroughly sprayed it with all kinds of aerosals to see if any idle changes result. So far there has been no idle response to my spraying.

It seems to me that the only way a vacuum leak could cause the symptoms I am seeing would be if it were rapidly opening and closing. If it were a steady vacuum leak, it would just give me lean AFR and low vacuum. Or I am I totally wrong on this one? I mean it really seems like it has to be something related to the valve train.

I am soooo at the end of my rope with this one. I about ready for an unexplained garage fire.
 
How about just what kind of vac readings are you getting? specificaly. what kind of numbers? drifting? or rapid movement? how much of a drop?


mark
 
Very rapid movements. Roughly 9.5-11 in. Hg at idle and 14.8-17.7 in. Hg at highway cruise.

BTW. Thanks to everyone who has replied. Even though I am
extremely frustrated with my car, I really appreciate all your feedback and suggestions.

And, Jack, I am sooooo tempted to hang a pair of SUs on this thing and forget about it. But I have invested sooo much time, thought and money on this EFI set up. I just can't give it up, yet. Also, I really don't think the problem is realted to my EFI. If I had multi-point injection, it might cause the symptoms I am seeing if an injector went bad. But even that is a long shot. I have a TBI injection which, in a lot of ways, is just a computer controlled carburetor. Nevertheless, vacuum fluctations like I am seeing would not be nearly as big a deal with carbs.
 
How steady is your timing mark at various rpms? I'm wondering about some other type of feedback causing the problem. Flakey TPS? Or do you just use MAP?
 
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