Question about possible BVSV problem

[charleyk]

I have an 86.5 NA which used to have a rough idle when starting hot which I could not correct. I attributed the problem to heat soak and left well enough alone.

While installing a new P/S pump, I accidentally broke a port off the BVSV on the thermostat housing. So, I bought a used one off the net and installed it.
Amazingly, although I have no explanation, the rough idle issue described above dissappeared. However, I'm now having a worse problem. Instead of a rough idle at start when hot, it doesn't start at all unless I wait about five minutes after a failed start to try again. Again, only when hot. This hot start problem is intermittent.

Does anyone know if the BVSV can in any way be responsible for this type of behavior? Could it be possible that the replacement BVSC while looking like the correct one is not correct?

Appreciate any advice someome may have with this as I don't want to waste time looking at the BVSV if it cannot have anything to do with this.

Thanks

 

[jetjock]

The BVSV was unlikely to be your previous idle problem unless the throttle plate is misadjusted. It could be your current idle problem if the same condition exists. It's more likely to be either a loss of fuel system rest pressure or a failure of the Fuel Pressure Up system.

Disconnect and plug the port on the TB that goes to the BVSV and see if the problem goes away. (Plug the hose too). If so check the TB and BVSV. You may also have a saturated evap cannister. If the hot start problem remains check the FPU system. Make sense?

You should not be able to blow through the BVSV when the the coolant is below about 110 F. You should be able to when above. The BVSV is connected to a ported source of vacuum on the TB. All that means is there's no vacuum on the TB port until the throttle moves a little off idle ie; the port is connected to a hole that's on the 3000 pipe side of the throttle plate when the plate is closed. Only when it opens is the port exposed to manifold vacuum.

So, the BVSV prevents the engine from sucking on the evap cannister when coolant is cold and the port prevents the engine from sucking on the cannister when the throttle is closed. Only when the coolant is hot and the engine off idle is the evap cannister purged. Got it?

To check rest pressure you really need a fuel pressure gage. No gage? Pop the plastic top off the pulsation damper and look at the little screw you'll find there. It'll stand up as long as there's fuel pressure. Shut the car off and watch how long it takes for the screw to drop. If it drops quickly, say withing 10 minutes, you're losing rest pressure. Not as accurate a test as using a gage but if the pressure loss is quick it'll tell you. Another way is to prime the fuel system by jumpering B and FP in the diag block just before a hot start. If things get better it's a sign pressure was lost.

Fuel Pressure Up system: Withe the engine hot there should no vacuum on the hose going to the fuel regulator for the first few minutes after start. If you don't have a vacuum gage simply disconnect the hose and see if the engine starts better when hot. If it does the FPU system is not working. Be sure to reconnect the hose afterwards.

 

[charleyk]

So you're saying that though it is highly unlikely , it is possible that a malfunctioning (or even incorrect?) BVSV could lead to the problems I described?

Thanks for the quick education and I will try these tests as the problem continues. Only thing is, I never know if the car will start on the next try or not making it difficult to determine if pulling a vaccum hose actually made a difference or not. I guess I may have to perfrom these tests as I go and correct items in order until the problem is corrected.

 

[jetjock]

Not sure I follow. The entire point of these tests is to pinpoint which, if any, of these systems are at fault. It the problem goes away the last system you played with is the culprit, especially if the car starts better several times in succession.

You gotta remember troubleshooting over the Net is largely a theoretical exercise. If the car were here I wouldn't use those methods because I have the knowledge and gear to quickly pinpoint the cause. My task on the forum is to come up with other methods the average owner can employ. The best way to do that is by using the car itself as a test tool.

Btw, I'm only talking about when the engine is hot because that's your complaint as I understood it. If the car does weird stuff when cold that's another story altogether.

 

[charleyk]

Got it. Well, It just happened again. Disconnecting the hose to the TB from the BVSV as suggested do not work. Just for the hell of it, I tried disconnecting the hose that goes to the evap cannister for the next crank and it started right up. I think that must just be a coincidence for if I understand you correctly, that should not have made a difference. Unless that points to a possibly saturated evap cannister?

 

[charleyk]

Next time it happens, I'll pull the hose to the evap cannister just to see if that makes a difference. If not, I'll keep going.

Thanks again

 

[charleyk]

It happened again. As expected, disconnecting charcoal cannister hose made no difference. By the time I moved on to next test, it started so I'll have to wait for next occurrence. Any other ideas anyone?