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Discussion Starter #1
Hey guys, I posted this under the "Bleeding Brakes", but nobody noticed it. As you recall, we are trying to find why the idle is so high (4000 rpm) on that B20F

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After flashing the tank with hot water, we installed the two plugs, and now the fuel pump did provide consistently good pressure. So, I attempted to diagnose the vacuum leak.

Turns out there are two sources. When I disconnected the manifold pressure sensor, the rpm dropped considerably. So, a large leak is from there. I was surprised the engine even runs without a MAP sensor. (that thingy on the right fender is the MAP sensor right, that's what I recall from reading the shop manual two weeks back).

Also, at the lowest part of the throttle plate, there is a big hole. When I plugged that with my finger, and with the MAP disconnected (and port plugged) the idle reduced to an almost reasonable 1400 rpm. I had the srew that adjusts the idle completely off. I assumed that this screw is a stop to the throttle plate, right? There was another hose right below that hole on the bottom of the throttle plate. I removed that hose too, but there was no vacuum there (I think that may have been the cold start thingy?

Anyway, is this MAP sensor available anywhere? Why did we have this vacuum leak under the throttle plate?
 

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The fact that the speed dropped when you unhooked the MAPSensor does NOT mean it is leaking vacuum (although it might be). You should check the MAPSensor for vacuum leaks by itself.
If it were mine I would solder the hole in the throttle plate shut. There should have been a spring-loaded valve over it hat was supposed to reduce emissions on overrun but mostly just made idle setting nearly or totally impossible, both on carbs and FI.
Be sure and do your idle adjusting with the 3/8" pointed bolt with locknut directly under the air intake on the manifold. Any other way messes up the TPS adjustment.
I have a few used MAPSensors. If yours turns out for sure bad, e-mail me offlist at the address below:
[username follows] gdowns [at] cableone [dot] net [nothing follows]
Include the last 3 digits of the p/n in your msg.
 

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Discussion Starter #4
Thanks Warlus and George.

Warlus, its possible that the MAP is not really leaking, its easy to blow through the hose and see what happens. However, there is no other explanation why the rpm goes up so much when you re-connect it. If anything, the sensor would think that the throttle is mushed down, and would dump more fuel, but in the absence of excess air, that would not increase the rpm, I would think. But anyway, I will blow through the sensor, and may be try one of your spares when we know for sure it leaks.

On the "hole". The hole I am talking about is not ON the throttle plate. Its a deep circular groove on the bottom of the manifold itself. You can see it on page 2-30 on the bottom of the page, the picture just above left of the bottom one. You can see a little hole, about 3/8 in diameter, just below the "throttle valve" (in fact, the arrow that shows the throttle valve intersects that hole). This is where I feel a bunch of vacuum leak, and when i plug that with my finger, the rpm goes down. In fact, with the MAP disconnected and its port plugged, I can feel a very healthy vacuum with the engine speed down to almost reasonable 1400 rpm. Why is that leaking air? I disconnected the hose just below that hole (don't know what that is for yet) and there was no vacuum leak there ... Need to understand that before I attempt to fix it.
 

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Discussion Starter #5
Well, I got the "new" sensor from Warlus (will send the old one back to you soon George), and there was no leakage from the diaphragm. I installed it on the car, and it made no difference. In fact, with the sensor off, the rpm was about 2500 (after I plugged another massive leak from a couple of hoses going to what seemed to be like a temp sensor on the water pump outlet), but with the sensor, the revs would go up close to 4,000. So, it seems like there is another gigantic vacuum leak, and its the lean mixture that prevents it from revving faster. Once I plug in the sensor, it gives more fuel to the engine and revs faster.

But where can be the vacuum leak? I have tried to plug the two vacuum hoses on the front left of the intake manifold plenum (the ones that go to the temp sensor), the one to the vacuum booster, and the one to the MAP sensor. I also tried to spray WD40 on the injectors, and that gave no difference either. So, what am I missing?

The other strange thing is that the engine blows a lot of smoke when it revs up. I doubt its rich mixture, if anything, it must be a bit lean. I am thinking that may be some of the piston rings are a bit stuck, and may be they will get loose once we bring the car on the road ... But we have to fix the gigantic vacuum leak first.
 

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"...lot of smoke..." "...gigantic vacuum leak..."

My guess is TWO problems - cracked intake manifold AND ruptured head gasket.

George Dill
 

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Discussion Starter #7
I was thinking about a cracked intake manifold ... But can that happen with the car just sitting? Is there a typical spot that these manifolds crack?

The head gasket ... I have replaced a head gasket before, so I am sure i can do it again.

Apparently, 20 years ago the water pump was gone on this car, and that's why its owner gave up on it. It is possible that the car was allowed to overheat excessively and killed the head gasket. However, 10 years ago, somebody replaced the water pump. But we see no water in the oil, and there is no misfire. We can easily do a compression test, a blown head gasket should show in a compression test. But before a head gasket can create such huge vacuum leak it would sure bring water into the oil first ...
 

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Do you have the idle adjusting screw (a pointed 3/8"-16 UNC bolt) directly under the intake installed and adjusted? If that screw/bolt is missing your idle will be very high. It is locknutted with a 3/8" hex nut (9/16" wrench) to make it stay where you put it. The tapered point on the bolt is over an inch long and it is slightly rounded on the end.
The screw that adjusts the throttle plate is NOT an idle adjustment, it is just to keep the throttle plate from binding due to closing too tightly. It is adjusted by closing the throttle and then adjusting the screw to where it JUST touches so it does not close any further.
 

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The best way I've found to set the throttle butterfly is to get a sliver of very thin paper about 1/4" wide, slip it under the almost closed butterfly, and then adjust the screw until you can just pull out the sliver of paper with some resistance. I find it better than using the thinnest feeler gauge I have.
 

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Discussion Starter #10
I have it now installed, but not adjusted. The shop manual was calling this idle adjustment, so I thought if it does not hold the throttle open, it won't matter. but I suspected that it not being there could cause a leak. So, now its on there, but not all the way in. I very seriously doubt that this could be causing this huge vacuum leak, especially now that its installed. I recall, prior to installing it (the threads were messed up, so it was off to clean the threads), I was putting my finger in and blocking the hole that the screw occupies. That action did bring the rpm down from about 3000 to 2500 rpm. However, once you install the MAP sensor, the rpm goes back to 4,000.

I asked the owner of the car, and back when his water pump gave, he did not overheat the car. So, he does not think that the head gasket is out. But a compression test is easy enough.

How about cracked manifold, do these intake manifolds have a specific weak spot that we can check?
 

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Try running the idle screw all the way down - don't tighten it, but install it all the way. Without that screw installed properly you do indeed have a massive vacuum leak that goes right past the throttle plate.
 

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If you don't have the steel bracket installed between the manifold and the block, the manifold and associated stuff are heavy enough to crack/break the cast aluminum. I have seen a couple of them broken. I suppose you have checked all the hoses and their fittings. Sometimes it is good just to plug all the openings in the manifold to make sure you haven't overlooked something. Do I understand correctly that you DO have the pointy 3/8"-16 idle adjusting stud installed in the manifold and screwed closed? There have been a couple of guys posted with fast idle that had lost the screw. (In fact I think I sent one to one of the guys.)
 

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Discussion Starter #13
Thanks Warlus and Lloyd.

Yes, that screw is on there. But its easy enough to tighten it all the way and try again, and see if it helps.

Yes, we have removed all the hoses one by one and plugged them (but not ALL together). There are not that many. One to the brake booster (no difference), one to the MAP sensor (this actually increases the idle even more now when plugged in, because apparently it gives more gas to the lean mixture due to vacuum leak), and two to the coolant temperature sensor up front.

I still cannot see how a bad head gasket can create a vacuum leak, but will do a compression test. May be the head gasket is responsible for the exhaust smoke.
 

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Also the thing you're calling the coolant temperature sensor is a cold start idle valve. If it's failed open it'll also be totally bypassing the throttle plate. Might want to pinch those hoses and see if it makes a difference.
 

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Discussion Starter #15
Yes, we removed these (there are two) and blocked the ports on the manifold. The front one had no vacuum, but the rear did. With these plugged, the idle was still very high. I think we tried all the ports we could see. Apparently, the hole in the MAP sensor was contributing, but it was tiny compared to the one we have now.

Warlus, I don't think anybody removed the intake manifold in the history of the car. But I don't know that for sure. Where is that steel bracket you are talking about? Under the manifold?
 

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Discussion Starter #16
Got another test run on the car. The screw on the front was now all the way in, yet the idle was still high.

I got it hot and did a compression test with all the plugs off. The numbers were not impressive. 105 all except 2 with 95. This engine has worse case scenario about 100K miles on it, so the cylinders and rings are probably in good condition. Is it sticky piston rings or head gasket? Still not entirely certain that a head gasket can create a huge vacuum leak, but given that we did not get a ridiculous number of 20 or 30 on one of the cylinders, I doubt the head gasket is causing the vacuum leak.

Good news is that I discovered one vacuum leak. Injector 3 is not properly installed. Is that enough to create such vacuul leak? Will find out next time.

Next step is to properly install #3 injector, and add a detergent to clean up those ring grooves. Any brand that anybody may have tried before for this purpose? I am thinking of STP ...
 

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Even a very small vacuum leak will cause a very high idle. I bet you'll have to turn the bypass screw back out some once the injector is in right.
 

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The steel bracket is BIG. It bolts to the bottom of the manifold and down to the side of the block. One bolt holds it to the block, 2 or 3 to the manifold.
 

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Discussion Starter #19
The steel bracket is BIG. It bolts to the bottom of the manifold and down to the side of the block. One bolt holds it to the block, 2 or 3 to the manifold.
The steel bracket is there, and no visible cracks on the manifold on the lower surface.

Also, I relocated the #3 injector, and that improved the high idle, but not enough. The injector reduced the rpm by about 500, but still races to 2800-3000 by itself. I removed the plastic black thingy on the passenger side of the intake manifold throttle body. That apparently is a variable throttle stop and allows higher rpm for a cold start. I sprayed a bunch of wd40 thinking that may be the throttle shaft is leaking. No difference. However, here is the super weird thing. I noticed there were two casted holes, about half inch in ID. When I plugged these with my finger, I did not feel any vacuum, yet a few seconds later the rpm started dropping. However, the rpm dropped not because the air leak was reduced, but because it started running lean. That was WEIRD. It may have something to do with the cold enrichment but it was still weird.

I think I will get a 500 hp V8 in this car, with twin turbos and nitrus injection. That should solve all the problems. May be a solid fuel rocket. But then I will have to upgrade the suspension. :)
 

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I think I will get a 500 hp V8 in this car, with twin turbos and nitrus injection. That should solve all the problems. May be a solid fuel rocket. But then I will have to upgrade the suspension. :)
I knew we would convert you to a hot rodder! LOL!!!! :)
 
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