Engine runs better with MAP sensor unplugged?

[Laser145]

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

[rfmaster]

Originally Posted by Laser145

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

When MAP sensor is unplugged ECM will set code 34 (MAP voltage too low) and reverts to a fixed fueling - significantly richer mixture than would normally be required. From your description it appears that you have fuel delivery issues. In other words volume and pressure of fuel delivered to injectors is insufficient under normal conditions.

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

[Laser145]

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!

[rfmaster]

Originally Posted by Laser145

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!

Each tank has its own fuel pump - so try switching from one side to another see if there is significant performance difference before and after replacing fuel filter. In other words measure fuel pressure before replacing anything, after fuel filter replaced - measure it again to see if there are differences between each side (port/starboard). Keep in mind TBI will run (idle) with fuel pressure as low as 6 PSI - but it will stumble and will have no power to speak of. Stock TBI really likes to have 14 to 17 PSI!

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

[Laser145]

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.

[rfmaster]

Originally Posted by Laser145

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.

The white smoke could mean big trouble - antifreeze getting into combustion chamber! Also, during cold morning starts there is a lot water in exhaust turning into steam, but this is probably not the case since it is summer.
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

[Laser145]

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...

[TheApocalyptican]

Just an idea: Have you made sure the vac line to the map sensor isn't plugged up at all?

[rfmaster]

Originally Posted by TheApocalyptican

Just an idea: Have you made sure the vac line to the map sensor isn't plugged up at all?

MAP Sensor has a dedicated vacuum port in the back of TB, between the two fuel lines. Nothing else should be plugged into that port.

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

[kalbert]

Originally Posted by rfmaster

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

I'll second that vote. Put a vacuum guage on the port for the MAP sensor and see what you get.

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?

[Laser145]

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!

[Laser145]

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...

[rs72z]

Fuel pressure regulator?

[rfmaster]

Originally Posted by Laser145

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...

The EGR solenoid is a PWM controlled by ECM. PWM = pulse width modulation. Based on your descriptions it sounds like you have original parts that have been subjected to 20+ years of use. The EGR valve take a beating and over time fail so this is not a surprise. You also may a crud filled EGR passage which may contribute to a stuck EGR. More than likely east coast humidity caused the EGR valve shaft to rust in one position. Make sure to save OE EGR - the information punched on its cover is very important to find correct replacement!!! A stuck EGR would effect idle.

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

[rfmaster]

Originally Posted by rs72z

Fuel pressure regulator?

Fuel pressure regulator is built into TB (in the back). Earlier in the thread FP was checked - appears to be around 13 PSI.

[TheApocalyptican]

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!

[rs72z]

Originally Posted by rfmaster

Fuel pressure regulator is built into TB (in the back). Earlier in the thread FP was checked - appears to be around 13 PSI.

He checked the fuel pressure before the filter, that doesn't rule out low pressure because of a faulty regulator.

[Laser145]

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?

[rfmaster]

Originally Posted by Laser145

I reset my fault codes and read them again... only reading "12".

Ideas?

Yes, read codes before resetting ECM! Plug MAP sensor back into harness, rev engine up and down wait for SES to turn on. After SES lights turns on and stays on while engine is running read codes (do not reset ECM!!).

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

[Laser145]

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?

Originally Posted by rfmaster

Yes, read codes before resetting ECM! Plug MAP sensor back into harness, rev engine up and down wait for SES to turn on. After SES lights turns on and stays on while engine is running read codes (do not reset ECM!!).

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

[rfmaster]

Originally Posted by Laser145

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?

After some head scratching on way back home I though of couple checks that you'll need to perform before anything else.

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

[Laser145]

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

Originally Posted by rfmaster

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.




//RF

[TheApocalyptican]

Not sure what kind of attachment you have for your pressure tester...but my inline one is installed permanently. That is to say, it can come off, but I'm not going to take it off. What I did to get mine on was to slightly bend the metal fuel line, but more importantly, I took the tbi unit off of the manifold to put the fuel test port on. Much easier to manipulate the entire throttle body to get the screws to line up with the manifold, than to get the stupid fuel line on a stationary throttle body.

[Laser145]

I took my TBI off as well... Still almost burned that truck to the ground before I got it all back on!
Posted via Mobile Device

[rfmaster]

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