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Engine - General, Exhaust & Induction Topics about the general engine items, exhaust system discussed here & Improving the intake tract - air filter to intake valve.

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Old 12-18-2010, 07:56 PM   #1
Overboost point too high?  
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Vilpo is keeping the peace
Ok, so i put in a manual boost control valve on my '89 Lebaron T2.
I noticed when i was dialing it in to reach the right pressure levels, that i could run it up to about 17-21 psi (1,5bar) momentarily without hitting the cutout. Now it only happened momentarily, since i immediatilly let off as i saw it going too high.. So does the boost need to hold there for a moment before it cuts out, or should it cutout right away when exceeding ~15 psi?
Now i know it does hit overboost at some point, since i hit it a couple of times before because i guess my WG solenoid wasn't working.
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Old 12-18-2010, 08:15 PM   #2
Re: Overboost point too high?  
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Fuel should cut right away. Once you go over 14.7 pounds, the computer no longer know how to compensate fuel with more air added, so it just stops the show.
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Old 12-18-2010, 08:20 PM   #3
Re: Overboost point too high?  
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do you have the boost controller on backwards????
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Old 12-18-2010, 08:21 PM   #4
Re: Overboost point too high?  
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or maybe if you have a grainger do you have the ball in backwards.
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Old 12-18-2010, 08:44 PM   #5
Re: Overboost point too high?  
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Yep, cut-out kicks in right away. You could just have a gauge that doesn't read correctly. Do you have another to swap in and try?
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Old 12-18-2010, 08:51 PM   #6
Re: Overboost point too high?  
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85boostbox is keeping the peace
if he has his boost controller hooked up wrong it would be like not having a vacuum line on your wastgate. Trust me from experience. And you know what happens when that happens. Car cannot see how much boost is going through it. Just check you boost controller to make sure it is installed correctly.
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Old 12-18-2010, 11:40 PM   #7
Re: Overboost point too high?  
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85boostbox your correct my ball was placed into the mbc at the incorrect end and i hit 20 psi. i blew the head gasket.
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Old 12-19-2010, 12:15 AM   #8
Re: Overboost point too high?  
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Do you have an MP puter? or maybe a custom puter? Maybe the OB timer is set higher. Some MP puters had the timer set higher or no overboost at all
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Old 12-19-2010, 03:26 AM   #9
Re: Overboost point too high?  
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hope this helps clearify the installation;
Attached Thumbnails
Overboost point too high?-g_valve.jpg   Overboost point too high?-mbc.jpg  
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Old 12-19-2010, 09:39 AM   #10
Re: Overboost point too high?  
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+1 on the computer. A stock SMEC will cut instantly and it will feel like hitting the end of a leash. If your MBC is installed incorrectly you can overboost, but that has nothing to do with why the car isn't cutting out. If you're seeing 20+ psi, then it can only be one of a couple things...

1) Non-stock SMEC. Check the part number on the chassis of the SMEC against the known part #'s here in the forums to verify what computer you have.

2) Inaccurate boost gauge. This isn't out of the question. Tee in a known good vac/psi gauge and test in the driveway for accurate vaccum. Close hood and take it around the block. Allow enough vac hose to rest/tape the gauge on the windshield and compare it to what your dash gauge is indicating. You might need a new gauge.

3) Kind of unlikely, but... you might have a 3-bar MAP sensor installed on a 2-bar SMEC. Again - check the part #. Visually, 3-bar MAPs tend to be longer than a 2-bar. If this is the case, your car wouldn't be running right, but the SMEC would be making enough adjustments in fuel delivery to compensate somewhat. You're running way lean if this is the case. I drove my '88 overnight from Denver, CO to Columbus, OH before I realized it had the wrong MAP installed. Luckily I drove it easy and the injectors were too large as well, helping to prevent too lean of a condition. I exceeded 16psi once, and that's when I knew something was wrong and took it easy the rest of the way.
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Old 12-19-2010, 12:46 PM   #11
Re: Overboost point too high?  
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+1

if you boosted past what SHOULD have triggered an overboost fuel cutoff,
but DIDNT get one ..
you have a problem in the MAP circuit, electrical or pneumatic.

fix it before you damage your engine.
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Old 12-19-2010, 01:39 PM   #12
Re: Overboost point too high?  
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Quote:
Originally Posted by Vilpo View Post
Ok, so i put in a manual boost control valve on my '89 Lebaron T2.
I noticed when i was dialing it in to reach the right pressure levels, that i could run it up to about 17-21 psi (1,5bar) momentarily without hitting the cutout. Now it only happened momentarily, since i immediatilly let off as i saw it going too high.. So does the boost need to hold there for a moment before it cuts out, or should it cutout right away when exceeding ~15 psi?
Now i know it does hit overboost at some point, since i hit it a couple of times before because i guess my WG solenoid wasn't working.
Overboost should kick in at 13.6 psi...
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Old 12-20-2010, 05:53 AM   #13
Re: Overboost point too high?  
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Vilpo is keeping the peace
Ok, thanks for the replies. So i'll clarify that i haven't had problems installing the mbc, now i'm running stock boost levels with it. The reason i asked because i think something is off since i was able to get boost past the OB cutout point.
Now i have checked the boost gauge to be accurate by t-ing a second pressure gauge next to it and it was very accurate up to about 11 psi (0,8bar), where i have it set now.
I will check the part numbers on the MAP and the SMEC, though they look stock to me. I was hoping to diagnose MAP by connecting my diagnostic tester on the dlc connector but for some reason im not getting it to connect.
So i'll propably check the wiring for the connector the same way as i check the SMEC part number.
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Old 01-03-2011, 04:48 PM   #14
Re: Overboost point too high?  
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Vilpo is keeping the peace
Ok, checked MAP part number is: 5227152. ECU number is: 5285 069. Though i think the ECU number could be also 5235 069, since it seems to match the number on my parts catalog, the numbers on the ecu were too faded to read properly... So they seem to be stock parts?
DLC connector seems fine, the wiring to the ecu seems good from my measurements with the DMM. One thing was a bit strange though, 2 pins were getting 11,3 volts, when my schematic said there should be only one pin getting voltage from ignition switch and the other shouldn't be in use even...
So i guess next i will check the MAP readings manually with DMM since i cannot connect with a tester.
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Old 01-21-2011, 04:24 AM   #15
Re: Overboost point too high?  
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Vilpo is keeping the peace
Ok, i tested the MAP sensor voltages during driving. First without engine running - 2.425v, then at 0,4bar boost (5.8psi) 3.4v and at 0,9bar (13,05psi) it was 4.52v. So there's a little error acording to the flow chart (lower voltage than it should be), but is it enough to make it not overboost shutdown at 1.5bar (21 psi)? I dont think so..
Though the plugs have been too white all the time, so could this be making it run a bit lean also?
Thanks for any help...
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