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Author
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Topic: 96 3.8L -- Misfire/Surging @ 1200rpm
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Pete Brubaker
New Member
Member # 2484
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posted
 
 
Hey all,
First let me brief you. This isn’t my car but I have been working on my own cars for quite some time. The car belongs to a friend at work (that doesn’t make much money) and it ended up needing the head gaskets changed. So I offered to help him out. He had taken it into the dealer to get a tune-up and service at 90k miles. He was experiencing surging/missing and a rough idle. They also found oil in the coolant suggested that it was a busted head-gasket. However we are still getting the missing/surging problem at or between 1100 and 1300rpm.
Description of the problem: The car runs great at idle and above 1300rpm and the acceleration is great. However when cruising at 25 - 35 MPH the car is right at around 1200rpm and it surges badly. I am assuming this is one of the cylinders misfiring. With the car running I throttled it by hand and I can hold it at a spot where it will cycle and drop 300rpm then rev back up over and over again.
What has been done you might ask?
-- New Head Gaskets
-- New Lower/Upper Intake Manifold Gaskets
-- New EGR Valve Gasket
-- New EGR Valve
-- New PCV Valve
-- New Valve Cover Gaskets
-- New O2 sensors on each header
-- New Air-filter
-- New Plugs
-- New Wires
-- New Ignition Coil
-- Tested MAF sensor
-- Tested TPS sensor
-- Tested the idle air control valve
-- Tested crankshaft position sensor
-- Tested camshaft position sensor
-- Tested rear O2 sensors (behind the cats)
-- Tested intake air temperature sensor
-- Tested coolant temperature sensor
-- Tested DPFE sensor (pressure feedback transducer)
-- Tested EGR valve solenoid
This RPM level is, incidentally, right where the computer changes spark advance timing, which is not adjustable. This is evident by viewing the timing and revving the car up. I know this timing is not adjustable but it appears that the engine is setting the timing at about 20 degrees BTDC at idle and 30 degrees BTDC while higher than 1100 rpm. The car idles fine at about 750 rpm.
The only thing I haven't checked yet is the fuel system.
One other thing to note, last night I un-plugged the MAF, and the TPS and forced the car into "limp" mode thinking that the car is having trouble choosing timing & fuel/air mixture. The problem still persisted.
Does anyone have any suggestions? Has anyone had a similar problem with their pony before?
Thanks,
--Pete
Posts: 6 | From: San Rafael, CA | Registered: Feb 2003
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Pete Brubaker
New Member
Member # 2484
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posted
I haven't done a compression check yet. I'll do that tomorrow night and post a reply.
I did however clean the injectors when I had them out with carb cleaner, and I replaced the o-rings where they seat into the head.
I just can't figure out why this would only happen in a specific RPM range.
--Pete
Posts: 6 | From: San Rafael, CA | Registered: Feb 2003
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Pete Brubaker
New Member
Member # 2484
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posted
Ok, well I ran a few more tests.
First thing today I scanned for any error codes. No codes.
I checked the fuel system pressure, and the fuel system pressure regulator. All good.
Next I checked the cylinder compression. All the cylinders read between 168 and 175 psi. Everything is good there.
Everything appears to be good.
The only thing I have yet to check are the injectors.
Any other suggestions?
Thanks,
--Pete
[ February 20, 2003, 07:56 PM: Message edited by: Pete Brubaker ]
Posts: 6 | From: San Rafael, CA | Registered: Feb 2003
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Pete Brubaker
New Member
Member # 2484
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posted
Is it possible that the PCM is bad?
--P
Posts: 6 | From: San Rafael, CA | Registered: Feb 2003
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