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Hunting engine speed at no load

  #16  
Old 11-01-2018, 12:08 PM
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update

The O2 sensor has 2 wires. Shouldn't be a ground problem.

I measured the temp sensor resistance at less than 50 degrees = 62kohm. I tried to start it with it unplugged, but it was too difficult. After letting it rough idle for a few minutes I pulled the connection and it sped up real smooth then quit. I measured 8kohm warm.

Sorry about the italics! Can't figure out how to turn it off with my phone. Mobile versions are a pain.
 
  #17  
Old 11-01-2018, 12:22 PM
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not really. Twin tanks are no problem. I've owned my XJ 6 since 1989, never a problem. My other twin tank Jag, a 1965 S type, no problem either. Stainless Steel tanks. Very simple system in the '65, very complicated system in the '84. But once you figure it out, it actually "fixes" itself. Problems start when you let the car sit, then you have to go through 20 thousand troubleshooting steps when all you needed was to add fresh gasoline. Same with Aerostars.
 
  #18  
Old 11-01-2018, 12:46 PM
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I finally got the V12 running after rebuilding 2 of the 4 carbs and switching to a cheap little aftermarket pump sucking out of a plastic gas can. It has a manual choke. You only get full choke while pulling it, so when you are holding the switch with one hand, the screwdriver with the other, (I lost the key, remember) you need a 3rd hand for the choke.

Back to the Aerostar. I put a couple gallons of fresh gas in and I've been thinking about filling it up, but I didn't want to in case I needed to drop the tank for something. Now the only reason I would need to would be to dump contaminants out. Fuel pressure is steady. Good drivability. I even tested the purge system by clamping the line to the manifold.
 
  #19  
Old 11-01-2018, 01:03 PM
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the Jag key:

www.britishcarkeys.com

I believe Peter can use the code you have to make you a key.
 
  #20  
Old 11-03-2018, 01:14 PM
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I might have it!

I decided to retest the fuel pressure with my own equipment. I didn't purge the line with the rental stuff, but it still shouldn't have acted the way it did. It acted like a compression tester with a check valve.

So with my own gage I 1st cleaned/purged the line and reattached the gage. I got 40psi immediately before engaging the starter. I then found that the pressure would drop to 35psi as the rpms rise with the throttle closed. This means that the regulator is responding like its supposed to, but might be overdoing it. 35psi might be too low for the injectors to work properly which causes the rpm to drop which raises the pressure which restores the rpm which drops the pressure, etc., etc.
 
  #21  
Old 11-04-2018, 02:10 PM
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Well there's nothing wrong with the fuel regulator. I disconnected the vacuum to it, fired it up with the same difficulty and it cycled just like before except at a higher and constant fuel pressure. I watched it respond to plugging and disconnecting the vacuum. The pressure is 40psi max dipping, at high rpm, to 35psi with the vacuum connected. The pressure is a constant 42.5psi while the rpm cycles with the vacuum disconnected.
So, while the rpm is misbehaving, the computer is making sure the A/F ratio is good. (The exhaust has been clean during all of these difficulties.)

So, I'm back to a vacuum related problem. Hard starting, crappy idle, excellent acceleration, and steady cruise.

One test I still want to do is heavy engine braking.at highway speed and creeping with foot off the gas. For years it has occasionally "galloped" in first (it's a 5-speed) with foot off the gas.
 
  #22  
Old 11-04-2018, 02:56 PM
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Earlier I wondered if the manifold leak closes when the throttle is open enough. It doesn't have to. See this article:
https://www.motor.com/magazine-summa...gement-system/
When the intake is at almost atmospheric pressure (like it is under open throttle) no air will pass through leaks so it's like there is no leak and engine performance is normal.
Damn! I was hoping I found an alternative to tearing the top off. Well, at least the gaskets are dirt cheap now.
 
  #23  
Old 11-05-2018, 01:23 AM
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That "gallop" is the EEC not properly controlling the engine, and the shock can break parts. I would try to stop that as soon as possible if it happens again, like releasing the clutch so it doesn't lurch the whole van.
 
  #24  
Old 11-05-2018, 10:34 AM
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Finally got around to measuring the manifold pressure. When rough idling it is around -5mg. As it warms, the rough idle rpm increases a little and the pressure drops to about -10mg. When rpm cycling, the pressure swings wildly, but never gets down where it's supposed to be. Major leak. Even though I sprayed the booster connection with ether before, I tried disconnecting it and plugging it. No effect.
 
  #25  
Old 11-05-2018, 05:04 PM
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Imagine the signals your MAP sensor is sending to the EEC. If I remember right the MAP sensor doesn't get it's vacuum from the tree but from a fitting in the intake manifold. I'd double check that hose and fitting. In fact, I'd replace that hose. I'd also take all the hoses from the tree and plug the fittings at the tree. Also inspect the tree and make sure it's not cracked or broken. You can even pull the brake booster hose and plug the fitting again. At this point you will have eliminated all the possible outside causes of vacuum loss, and you know the rest of the story.
There is one other thing that can cause these symptoms. A leaking head gasket BETWEEN two adjacent cylinders. I'd do a compression check before tearing into the manifold gaskets.
 
  #26  
Old 11-05-2018, 05:37 PM
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You better believe I checked and double checked all those lines! A lot easier than tearing it apart. A blown head gasket will cause drivability effects, water in the pan, etc., none of which I have.
Parts are on order. Should get 'em next week.
Hopefully the manifold isn't cracked.
 
  #27  
Old 11-06-2018, 09:23 AM
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My parts are scheduled to arrive the end of this week. The 4 dollar gasket kit includes the gasket for the bypass valve which I have been reusing too many times. I'm confident that the old gasket is still sealing the perimeter, but I can't be as sure about leakage underneath from port to port. So, before diving in, I'll try just replacing that gasket.
 


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