Same problem new motor..

[ChevyTech]

Originally Posted by TrueChevyTech

I've read many of your posts and would like your opinion, my thread http://67-72chevytrucks.com/vboard/s...d.php?t=500255 , also I did not plan any relation of our forum names and will change mine if you suggest

Your user name does not bother me.

You have gotten a lot of good advice already.
You set the timing using the correct procedure, and the fuel pressure has been tested and is good.

Looking at the EGR was a good idea.

I have covered EGR systems in great detail in the past on the 88-98 truck board and you can search for my posts there.
Here is a post that many members have told me they found very helpful:
http://67-72chevytrucks.com/vboard/s...d.php?t=347247

Originally Posted by 79k30dually

The vehicle speed sensor input is a controlling option for the EGR. Ron

Originally Posted by 79k30dually

disconnect the vacuum to the EGR valve and plug it, then retest. The ECM doesn't open egr valve until the truck is moving.

There are so many variables on the EGR systems used on GM trucks, and this is true for some of the newer systems but not for a 1987 TBI ½ ton with a negative backpressure EGR system. I am assuming it is a ½ ton he has.

TBI trucks are generally Negative pressure EGR or Port EGR systems.

There are multiple types of EGR valves and system used on these trucks.
MANY PEOPLE DO NOT REALIZE THIS.

Positive back pressure EGR valves - Should have a “P” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Negative pressure EGR valves - Should have an “N” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Port EGR Valves – 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 valve.

Linear EGR valves – Electric solenoids on valve.

Originally Posted by TrueChevyTech

Yeah slightly above idle the egr opens all the way, …..Posted via Mobile Device

There is not enough vehicle information for me to be SURE what EGR system your trucks has but most light duty early TBI trucks have a negative backpressure EGR system.

The EGR valve should not be opening all they way, with the throttle just off idle.

On negative backpressure EGR systems the ECM powers the EGR solenoid when the engine might need EGR operation and it is the internal mechanism in the Negative backpressure EGR valve that modulates how much the EGR valve opens.

EGR valves have a spring in them that can get weak, or break, causing the valve to open too much or to easily.

Here is a site with a good illustration of a negative backpressure EGR valve.
http://shbox.com/1/EGR_valve.jpg

Negative pressure EGR valves - Should have an “N” after the last numbers on the valve. The pressure in the exhaust system effects (modulates) how much the valve opens.

Negative backpressure valves should hold vacuum with the engine off. When the engine is started the valve should bleed vacuum. The slightest pulse of pressure in the exhaust system will modulate recirculation (close the valve at least some). A good test for these is to put a hand vacuum pump on the EGR valve and apply vacuum with the engine off and it should hold vacuum. Leaving the vacuum on it, start the engine, and the valve should close.

If you find it does need an EGR valve, I would suggest only using a general motors OEM (Original equipment manufacturer) replacement EGR valve.

You mentioned your exhaust system:
Exhaust system backpressure can greatly effect the operation of the EGR valves on many vehicles. Modifying the exhaust system can effect how well the EGR works if it changes the pressure in the exhaust system. Exhaust back pressure (from a restricted converter) or no back pressure (from no converter or mufflers) can have an effect on how much the EGR opens.

That being said many people get by with exhaust modification on negative backpressure EGR vehicles.

If you give me the EGR number I may recognize it, but with out more vehicle information it is only a guess that your truck has the negative back pressure EGR system.

I will stop here so this post does not get to long. Let me know if you have more guestions.

The REAL “ChevyTech”