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#1 |
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Location: Brooklyn, New York
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Engine runs better with MAP sensor unplugged?
Spent my first day with my new truck fishing through a few issues...
Details: 87 350 TBI The truck idles very well. If you're extremely gentle on the accelerator then it drive fine... If you apply much gas at all, the truck bogs down hard. Noticed the map sensor was unplugged, strange... plugged it in and it made the idle loopy and made the truck run much worse. I had a new map sensor...plugged it in, same thing. I haven't replaced the fuel filter yet, but I will be Saturday. Any ideas on the MAP sensor making things worse? Cheers
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#2 | |
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Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
Quote:
1) Pull set DTC codes by installing a wire jumper between pins A&B of ALDL connector and read SES light blinks. With MAP open Code 34 should be set. Additional codes might be present as well. 2) Check fuel pressure - stock TBI pressure is spec'd between 9 to 13 PSI. If it on a low side or below 9 PSI - fuel pump R&R is most likely required. 3) Injectors spray pattern contains large droplets and uneven? Dirty injectors with a combination of low fuel pressure may be the cause. 4) Replace Fuel filter - when was the last time you replaced fuel filter? 5) Last but not least - after 23 years in service fuel pumps wear out or the outlet port to feed through fuel line leaks and starves injectors. //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#3 |
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Location: Brooklyn, New York
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Re: Engine runs better with MAP sensor unplugged?
1) I have a fault code reader... its only reading 12.
2) Fuel pressure I will be checking tomorrow. 3) I'll check out the injectors 4) Replacing fuel filter tomorrow 5) Fuel pump will be my last resort... Question... Are there 2 fuel pumps for dual tank trucks? Thanks for the direction!
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#4 | |
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Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
Quote:
Before even considering FP replacement measure fuel pressure with a gauge. If FP replacement is required go with AC-Delco Part # EP386 (25168719, AEP386). Good luck - let us know. //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#5 |
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Location: Brooklyn, New York
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Re: Engine runs better with MAP sensor unplugged?
Today I replaced the fuel filter (it was nasty inside). I also hooked up a pressure gauge just before it and got just over 13PSI from each of the pumps... both at idle and throttle.
The fuel filter did not fix my problem. The truck has new plugs. Today I bought a new cap, rotor, and wires, I'll instal tomorrow. It also smokes. Some white puffs at startup, then settles down while driving...however, if I let it idle for awhile the smoke gets worse, worse, worse, worse... until its really very bad. Tomorrow I'm going to check the compression too.
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#6 | |
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Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
Quote:
Warm up engine and pull plugs, number them and inspect. With plugs out run a compression check - 10% delta between cylinders is normal. EFI engines generally have very clean plugs even after going for many miles. A little bit of gray-brown ash deposits is normal from fuel additives. http://www.classictruckshop.com/club...park/plugs.htm If compression check is OK install new plugs and ignition components. Check and see if you are loosing coolant and with engine cold remove radiator cap and look for bubbles in the coolant flow as engine warms up. As engine RPMs are increased there a corresponding bubble increase would indicate coolant leak into combustion chamber. Other things to check - engine oil if milkshake-colored oil; overheating; rough running; spark plug(s) that have a green tint (if green coolant); white-colored or sweet-smelling exhaust. Overtime antifreeze in exhaust will poison O2 sensor. //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#7 |
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Re: Engine runs better with MAP sensor unplugged?
Compression check went fine... around 150psi in every cylinder at idle... around 175 at WOT.
Replaced the ditributer cap rotor and wires too... One of my plugs was loose, real loose... MAP sensor cause idle to lope heavily, and causes much more stumbling while driving. When it's unplugged it idles great but acts like it's far too rich when you step on the accelerator hard...
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#8 |
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Join Date: Jun 2010
Location: Las Vegas, Nv
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Re: Engine runs better with MAP sensor unplugged?
Just an idea: Have you made sure the vac line to the map sensor isn't plugged up at all?
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1995 Suburban K2500 454.....off road & towing 1999 Astro, work van 2006 Silverado 2wd 5.3 w/4.5" lift and 33s |
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#9 | |
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Join Date: Oct 2003
Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
Quote:
Sanity check. Get a vacuum gauge - measure idle vacuum at MAP vacuum port (back of TB). The LO5 (350) engine should have around 18 to 20 in-Hg intake vacuum, at idle (600 RPM), which should be applied to the MAP sensor inlet port. Now, if you have a vacuum leak - due to a bad hose or bad TB port, the MAP will see lower vacuum which corresponds to engine operating under load. The ECM will add more fuel when engine really does not need it. That may explain rough engine running. //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#10 | |
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Join Date: Jun 2012
Location: Minnesota
Posts: 509
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Re: Engine runs better with MAP sensor unplugged?
Quote:
It's very possible there is a vac leak somewhere. Rotten rubber hoses, bad intake or base gasket, stuck EGR valve, maybe even a bad brake booster, who knows. All easy stuff to check. It's a little odd that unplugging the MAP doesn't set any code, just for fun have you tried unplugging the coolant temp or something to see if that sets a code? |
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#11 |
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Location: Brooklyn, New York
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Re: Engine runs better with MAP sensor unplugged?
I'm still learning about these systems as I go. I've got a vac gauge and will start checking the lines today. I can see some of them are dry rotting, so I'll buy more and replace as I go.
The truck is running a little better now... one plug was loose, new cap, rotor, wires, etc... but it still feels a little rich. Thanks for the help so far!
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#12 |
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Re: Engine runs better with MAP sensor unplugged?
Pretty sure my EGR valve is stuck, also the solenoid may be out...
There is vacuum from the front of the TBI into the EGR solenoid. There is no vacuum from the solenoid to the EGR valve... Also, I can't move the diaphragm with my finger... I replaced cracked vac hoses today and hooked my pressure gauge up everywhere I could find... everything read 19, so that's ok...
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#13 |
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Location: Emory TX
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Re: Engine runs better with MAP sensor unplugged?
Fuel pressure regulator?
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___________________ 78 ccsb dually 5.3 4l60e http://67-72chevytrucks.com/vboard/s...d.php?t=491551 |
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#14 |
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Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
Fuel pressure regulator is built into TB (in the back). Earlier in the thread FP was checked - appears to be around 13 PSI.
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#15 |
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Join Date: Jun 2010
Location: Las Vegas, Nv
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Re: Engine runs better with MAP sensor unplugged?
The valve diaphragm should move fairly easily.
Just as a fyi....the solenoid only activates at certain times. If you are testing at idle....there should NOT be vac from the solenoid to the valve, only from the tb to the solenoid. You'd have to get the rpms up to see any vac coming out of the solenoid to the valve. Won't do much good if the valve is stuck tho!
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1995 Suburban K2500 454.....off road & towing 1999 Astro, work van 2006 Silverado 2wd 5.3 w/4.5" lift and 33s |
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#16 |
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Location: Emory TX
Posts: 900
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Re: Engine runs better with MAP sensor unplugged?
He checked the fuel pressure before the filter, that doesn't rule out low pressure because of a faulty regulator.
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___________________ 78 ccsb dually 5.3 4l60e http://67-72chevytrucks.com/vboard/s...d.php?t=491551 |
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#17 | |
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Join Date: Oct 2003
Location: OC CA
Posts: 1,374
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Re: Engine runs better with MAP sensor unplugged?
Quote:
Check EGR solenoid coil continuity by disconnecting harness plug and probing with DVM. I do not recall coil resistance right of top, but it should be between 30 to 60 Ohm. They do go bad occasionally - SMP VS12 (14102002, 214-329). //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#18 |
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Location: Brooklyn, New York
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Re: Engine runs better with MAP sensor unplugged?
EGR valve replaced...no change.
New MAP sensor plugged in...still runs worse, no change. Checked continuity on EGR solenoid... read mid 40's I'll check the fuel pressure at the TBI next. Just to recap.... The truck idles and handles gentle throttle perfectly when the MAP sensor is unplugged. When I plug in the MAP sensor the idle lopes up and down, and the truck sputters and misses in ALL throttle positions. Also the check engine light comes on. These conditions only exist under load. If I'm in park and just rev it up it seems to rev fine. I reset my fault codes and read them again... only reading "12". Ideas?
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#19 | |
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Re: Engine runs better with MAP sensor unplugged?
Quote:
You should get first a group three code 12, followed by groups of three of any other codes that were detected. Lets do this before throwing any more parts into it. //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#20 | |
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Location: Brooklyn, New York
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Re: Engine runs better with MAP sensor unplugged?
I reset the ECM today, plugged in the MAP sensor, drove the truck around the block, then read the codes... still just reading "12"
On another note...I'm losing tranny fluid from where the driveshaft enters the tranny. Isn't there a seal there? Know the name of it? Quote:
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#21 | |
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Join Date: Oct 2003
Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
Quote:
It has to with MAP circuit wiring. Engine off, ignition off. Disconnect MAP sensor plug. Get a DVM - set to read Ohms. Check ground circuit continuity between pin A of the MAP plug (purple wire) and good electrical ground. It should be close to 0 to 2 Ohms max. If it is open - i.e. very high resistance this would explain why MAP is not working correctly. Lets do this simple check before getting anything else replaced. Rear trany seal - assuming that you have 2WD 700R4 - there are couple of seals that will work. There is an updated seal used in 4L60E starting 1996 and up that will retrofit into 700R4. Can not recommend part numbers since I do not know your trany type! (timken 9613S; National 4583 ). But read this thread first: http://ls1tech.com/forums/automatic-...ing-again.html //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#22 | |
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Location: Brooklyn, New York
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Re: Engine runs better with MAP sensor unplugged?
I think I did this test the other day... but I've done so many I'm confused! I'll do it again just to make sure.
I've now replaced the MAP sensor and the O2 sensor. Didn't change a thing. I'm getting code 33 for MAP sensor and code for O2 sensor/rich exhaust (I think it was 45, maybe 44...don't remember). Truck now coughs, struggles, and lopes when first cranked... after about 5-8 seconds it straightens out and idles perfectly. Runs out fine as long as I'm soft on the gas pedal... misses, hesitates, struggles if I stomp on it... but eventually overcomes and accelerates. The only thing I haven't checked that we've talked about is fuel pressure at the TBI... I tried so hard! The metal lines just don't give me enough clearance to hook up my inline pressure gauge... I barely got the fuel lines back on without the pressure gauge, much less got it hooked up. Not sure how I'm going to do that... may try to tackle it again when I'm feeling patient. So I'm at a loss... Maybe I missed a vacuum line somewhere? everything I could find I replaced... they all read fine anyway. Cheers Taylor Quote:
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#23 |
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Re: Engine runs better with MAP sensor unplugged?
I took my TBI off as well... Still almost burned that truck to the ground before I got it all back on!
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#24 |
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Location: OC CA
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Re: Engine runs better with MAP sensor unplugged?
There are two trouble codes associated with the MAP sensor. A code 33 will set if the MAP sensor output voltage is higher than expected (which indicates low vacuum) and a code 34 will set if the MAP sensor output voltage is lower than expected (indicating high vacuum).
As with any trouble code detected, you should not assume the presence of either one of these codes indicates the MAP sensor itself is bad. All electrical and vacuum connections to the MAP sensor should be verified before replacing the part. To give a couple examples of what to look for should you get a code 33 would be a vacuum leak, mechanical issue with the engine causing very low vacuum levels, or an electrical problem between the sensor and ECM. If you are getting a code 34 you should look for a collapsed or blocked vacuum line going to the MAP sensor or electrical problem between the ECM and sensor. Normal output voltage of a MAP sensor should be about 4.5 volts or so with key on, engine off; and less than 1.5 volts with engine running at idle. You can measure MAP sensor output voltage by 'back probing' pin B of MAP connector. Procedure: 1) Configure Multimeter to read Volts DC. 2) Ground the BLACK Multimeter Test Lead directly on the Battery Negative Terminal. 3) Turn the Key on but don’t start the Engine. This will enable the MAP Sensor to get power in the form of 5 Volts from the ECM. 4) With the Key On and the Engine Off and without any Vacuum applied to the MAP Sensor your Multimeter should register between 4.5 to 4.7 Volts DC at Pin B (connector plugged into MAP sensor). To measure you will have to back probe through the rubber seal through the same opening used by the light green wire. You can use a small diameter wire or unfolded paper clip. It is difficult to make contact sometimes! 5) If you have a handheld vacuum pump (Miti-Vac) you can verify that MAP works correctly across vacuum range: 0 in. Hg ...... 4.7 Volts. 5 in. Hg ...... 3.9 Volts. 10 in. Hg .... 3.0 Volts. 20 in. Hg .... 1.1 Volts. Stock engines usually idle between 1.3 and 1.6 Vdc. Question When was the last time you checked timing????? //RF
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"The Beast" 1975 Chevrolet C20 longbed 350/700R4! with 3inch body lift Dual Flowmasters Super 40's! TBI retrofit completed (2007-07-29) ![]() New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17) |
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#25 |
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Re: Engine runs better with MAP sensor unplugged?
I've done the test where the voltage drops with my vac gauge. Came out perfect.
Timing I checked a month ago. Spot on... Seriously though... Thanks for all this help I'm learning a lot! Posted via Mobile Device
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