Idling issue on 1988 C3500

[Rocky98Formula]

Hi there,

I have a 1988 C3500 pick up with the 350 TBI engine and 3 speed auto transmission. It's a seasonal farm vehicle and I drive it around 6 months out of the year in spring/summer. For the last couple years, I've been trying to troubleshoot the following issue. When the engine is warm and I come to a stop at a red light, the engine will run at idle for 5-10 seconds before it just cuts out all of a sudden. There is no shuddering or misfiring before the engine dies. IF I drop the gear into neutral when I come to a stop, it will stay running until the light goes green at which point I can pop it into gear and drive off. If left in drive, the engine will shut off every time. It used to throw a code/engine light right before it would stall, but now it does not. No codes, no lights, just shuts off. I took it in to a mechanic last year and they changed out the plugs, plug wires, rotor/distributor cap/ignition module and the coolant temp sensor annd the 1-wire O2 sensor. Prior to this, the light would come on and then it would stall. After this, no light but same stalling.

I've checked the temp sensor. It reads approx 2900 ohms at 25C and 250 ohms at operating temperature. This appears to be in spec. I probed the leads with a multimeter and was getting approx 3.5 volts when cold and 1.0 volts when at operating temp.

I've removed the TBI and cleaned out every port. It was cleaned previously by the mechanic and was still fairly clean. I've replaced all the vacuum lines that lead from the TBI. They all looked fine but I replaced them anyways. I've confirmed via the paper clip method, that the engine stays in closed loop mode once it's warm and at a stop. In general, I get about 11 MPG combined city and highway driving.

Every time the engine stalls, it will start back up with no issue. As mentioned above, it will stall if left in drive at a red light. It also stalls almost every time I coast down the 2 block hill to my house and turn left into my driveway. When driving, it runs strong and smooth. When I start 'er up in the morning, she will run strong and idle just fine in gear. After warming up, the stalling begins.

I cannot confirm if the O2 sensor and temp sensor are original OEM or aftermarket. I've read that non OEM sensors can cause issues. Also, I am noticing a puff of white smoke at startup but nothing else after the first 2-5 seconds.

For under $300, I think I can purchase a new OEm temp sensor, MAP sensor, EGR valve, EGR solenoid, and IAC valve from sites like RockAuto or Amazon. Do you think this is a good first step or should I replace sensors one at a time? I love this truck and would like to keep her going for years to come. I would love some input from others with similar issues before going and swapping out everything I can.

Thank you kindly,
Rocky

[oldertruckfan]

how many miles on it ? have you replaced the fuel filter lately ?

[ChevyTech]

Hi Rocky, and welcome to the message board.

It is true that low fuel pressure affects the idle on TBI engines. Oldertruckfan has a valid point.

Don't replace parts unless they test out to be bad. The quality is so bad on replacement parts that you don't want to try to fix it by throwing parts at it.

The coolant sensor readings you got were good so don't replace it.

It sounds to me like the IAC (Idle Air Control) is not working. If the TPS (Throttle Position Sensor) does not give the ECM a correct voltage for idle the ECM will not send pulses to the IAC to control the idle speed.

TPS testing with a meter:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.

If the TPS test out good, I would be suspicious of the IAC. They get old and stick. Carb cleaner can damage them.

If there is a part I would say to try if you want to try parts, it would be the IAC.

If the EGR is sticking open it can cause it to stall.
You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.

You could try to lift the EGR diaphragm with your fingers, with the engine idling, (being careful not to burn yourself or get your fingers caught in the EGR valve) and see if it closes well or wants to stick open.
NOTE: Your 1 ton truck may not have a negative backpressure EGR valve like the smaller trucks and there might not be big enough openings to lift the diaphragm by hand.

If you find it needs a EGR valve, lets talk it over before you pick one to buy.

Does your truck have a fast idle when you start it and then the RPMs slow back down as it runs?

Do you have to push, or hold down, the gas pedal to get it to start?

[oldertruckfan]

i'm all ears on the EGR valve and which one to buy. thank you Chevy tech! I just picked up a 95 K3500. its in here somewhere

[ChevyTech]

Most of the 1 ton trucks have a Port EGR valve in which the EGR solenoid pulses like a mixture control solenoid/fuel injector to control vacuum supply. Port valves have no “P” or “N” and it is blank after the last numbers on the Origional factory valve.

The testing is much different then a ½ ton negative backpressure system.

If you verify it does have a Port EGR system, then the EGR valve you buy is not so critical because it is a simple diaphragm.

Negative backpressure valves have more going on inside then people realize.
The TBI trucks with this valve are very picky about its operation and you want to stay with what is came from the factory with.
Here is a site with a good illustration of a negative back-pressure EGR valve.
http://shbox.com/1/EGR_valve.jpg

The easiest way to identify a Port system is by the EGR solenoid. This thread has a photo of a Port EGR solenoid.
http://67-72chevytrucks.com/vboard/s...48#post4083748

I kept this as short as possible not to highjack his thread and make it to long, but I thought he may be wondering the same thing.

Truckfan you can send me a private message with a link to a thread of yours and I will talk more there.

Posting photos really helps me, help you.

[Rocky98Formula]

Originally Posted by ChevyTech

Hi Rocky, and welcome to the message board.

It is true that low fuel pressure affects the idle on TBI engines. Oldertruckfan has a valid point.

Don't replace parts unless they test out to be bad. The quality is so bad on replacement parts that you don't want to try to fix it by throwing parts at it.

The coolant sensor readings you got were good so don't replace it.

It sounds to me like the IAC (Idle Air Control) is not working. If the TPS (Throttle Position Sensor) does not give the ECM a correct voltage for idle the ECM will not send pulses to the IAC to control the idle speed.

TPS testing with a meter:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.

If the TPS test out good, I would be suspicious of the IAC. They get old and stick. Carb cleaner can damage them.

If there is a part I would say to try if you want to try parts, it would be the IAC.

If the EGR is sticking open it can cause it to stall.
You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.

You could try to lift the EGR diaphragm with your fingers, with the engine idling, (being careful not to burn yourself or get your fingers caught in the EGR valve) and see if it closes well or wants to stick open.
NOTE: Your 1 ton truck may not have a negative backpressure EGR valve like the smaller trucks and there might not be big enough openings to lift the diaphragm by hand.

If you find it needs a EGR valve, lets talk it over before you pick one to buy.

Does your truck have a fast idle when you start it and then the RPMs slow back down as it runs?

Do you have to push, or hold down, the gas pedal to get it to start?

Hi ChevyTech, thank you for your response and thank you to everyone else as well.

In response to Oldertruckfan, the truck has about 160k miles on it and I should have mentioned that the fuel filter was replaced about 5k miles ago.

ChevyTech, I tested the TPS as per your method and got a value of 0.68 volts at idle and it swept smoothly up to 4.20 volts at full throttle. I removed the IAC valve and inspected it. The internal surfaces are all clean but it does appear to be the original factory valve. I will order one and replace it as seems like a good place to start as you also confirmed.

I checked the part number on the back of the EGR valve and it read 'DBR17087185 118 72'. The EGR solenoid also looks similar to the one you posted a link to. Based on these 2 items, I believe you are correct in saying that this 1 ton truck has the Port EGR system. There is enough room to get my fingers on the diaphragm but I did not try any tests with the engine running as I don't know what to look for to see if the EGR valve is working as expected. Should I replace the IAC valve first before looking at replacing any of these EGR components? I have to order OEM parts online as the local shops all seem to carry aftermarket brands, and I'm wondering if I should just order both at the same time.

In the mornings, the truck has a fast idle when first started and will slow down a bit as the truck is driven. I never need to touch the gas pedal when starting the truck cold or warm as it will fire up quickly by just turning the key.

[mountie]

New vehicles have many sensors to make start up a nice idle. My '88 350 TBI only has the oxygen sensor to perform that. So, at start up, the idle is sort of high, only for a short time..... as soon as the sensor determines the temp, then it lowers the idle to a correct level. When I had similar idle/RPM/ drivability issues, I discovered ALL THE ENGINE GROUND CONNECTIONS needed cleaning ( inc. computer chip 'tangs' ).

Bad grounds give bad information to your computer & sensors.

When I cleaned all the grounds, my idle problems went away.

( anyone with an older vehicle )

[Rocky98Formula]

Originally Posted by mountie

When I had similar idle/RPM/ drivability issues, I discovered ALL THE ENGINE GROUND CONNECTIONS needed cleaning ( inc. computer chip 'tangs' ).

Bad grounds give bad information to your computer & sensors.

When I cleaned all the grounds, my idle problems went away.

( anyone with an older vehicle )

What are the computer chip 'tang' you refer to? Thank you.

[ChevyTech]

You're welcome Rocky!

The TPS voltages looks good.

In the mornings, the truck has a fast idle when first started and will slow down a bit as the truck is driven. I never need to touch the gas pedal when starting the truck cold or warm as it will fire up quickly by just turning the key.

Then the IAC is working at least some because it is used to get the fast idle when started cold.

If the IAC did not work at all they tend to load up, or flood, when you try to start them, if you do not hold the gas pedal down.

The IAC still could need to be replaced, and I can't make the call without being there. We will not rule it out yet.

I would not order any parts yet. No part stands out as being the problem for sure.

The truck in the link I posted for photos has the same part number on the EGR valve as your truck. Make a note for future reference that your truck has a Port EGR system. Very few poeple will know how to test it.

I would test the fuel pressure, but it is not easy because there is no test port on these TBI trucks and the pressure should be tested with the engine running.

I would look in the distributor.
On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

If there is oil coming up the shaft and getting out onto the pickup coil – replace the distributor.

These distributors can also have problems because of loss of magnetism needed for the pickup coil to work.
If the magnetic material on the shaft in the reluctor wheel area is cracked into pieces replace the shaft or the whole distributor.

Photo of cracked magnet in this thread:
http://www.chevytalk.org/fusionbb/sh...e0b728a96e45b/

Keep looking until we find something that looks like a problem. Your communication skills are great so we should keep working at this instead of taking a guess and buying parts.

[Rocky98Formula]

Originally Posted by ChevyTech

You're welcome Rocky!

The TPS voltages looks good.

Then the IAC is working at least some because it is used to get the fast idle when started cold.

If the IAC did not work at all they tend to load up, or flood, when you try to start them, if you do not hold the gas pedal down.

The IAC still could need to be replaced, and I can't make the call without being there. We will not rule it out yet.

I would not order any parts yet. No part stands out as being the problem for sure.

The truck in the link I posted for photos has the same part number on the EGR valve as your truck. Make a note for future reference that your truck has a Port EGR system. Very few poeple will know how to test it.

I would test the fuel pressure, but it is not easy because there is no test port on these TBI trucks and the pressure should be tested with the engine running.

I would look in the distributor.
On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

If there is oil coming up the shaft and getting out onto the pickup coil – replace the distributor.

These distributors can also have problems because of loss of magnetism needed for the pickup coil to work.
If the magnetic material on the shaft in the reluctor wheel area is cracked into pieces replace the shaft or the whole distributor.

Photo of cracked magnet in this thread:
http://www.chevytalk.org/fusionbb/sh...e0b728a96e45b/

Keep looking until we find something that looks like a problem. Your communication skills are great so we should keep working at this instead of taking a guess and buying parts.

I opened up the distributor cap to have a closer look inside. As I mentioned above, the entire assembly was replaced last year. I don't see any play in the shaft and the wear on the cap itself is minimal. I did notice that some of my ground points looked corroded. The thermostat ground was already cleaned by my mechanic last year and I've gone ahead and cleaned up the alternator harness ground point and replaced the ground cable from the battery to the engine block as I had a new cable from a previous project on hand.

One thing I did notice was that the coolant level in the radiator was down about one liter. I topped it up about 3000 miles ago and it needed topping up again today. I don't have any external leaks and my oil does not appear to have water in it. Could this indicate a bad intake gasket and if so, could that contribute to stalling at idle?

I think I can borrow a tee adaptor to check the fuel pressure. I would like to rule it out and will report back once I check the pressure

Thank you once again,
Rocky
Posted via Mobile Device

[ChevyTech]

Leaking head gaskets can cause lose of coolant into the combustion chambers. Pressurizing the cooling system with the spark plugs out can reveal a head gasket leak into the cooling system. It takes some time but coolant can leak into a cylinder and come out a spark plug hole.

When the head gaskets fail between the two center cylinders, which is more common with age, these trucks tend to ping and the electronic spark timing system will retard the timing. There is a noticeable lack of power when this occurs. A failed head gasket leaking between cylinders shows up with a compression test.

The bottom photo, once again, confirms your truck has a Port EGR system. The module on the other side of the bracket from the EGR solenoid is the spark control module which is between the knock sensor and the ECM.

The distributor looks nice.

If the fuel pressure tests to be good, I would be trying a different IAC.

[mountie]

Originally Posted by Rocky98Formula

What are the computer chip 'tang' you refer to? Thank you.

Your computer chip plugs onto the circuit board. The metal 'plugs' , I call the tangs. They can corrode. Like a bad ground.

( I am sure there is a 'tech' word for those sharp metal things)