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”

[TrueChevyTech]

Egr valve seems in good shape, how much vacuum should it take to open it? And why would it open without the solenoid letting vacuum to it, this thing leaves me scratching my head
Posted via Mobile Device

[TrueChevyTech]

Yes the egr has a N, so negative back pressure it is. The numbers on the valve are "dcw 17087198 086 71N" it is 87 half ton 2wd , factory 305, previous owner did a 350 swap, it ran great for 4 years for me then after sitting a couple months it started stumbling slightly above idle. Moving or parked.
Posted via Mobile Device

[ChevyTech]

The EGR should not open at all without vacuum from the solenoid.

Sometimes the spring inside the EGR valve gets rusty and breaks.

It takes very little vacuum to move the EGR valve diaphragm.

Did you look at the link of the illustration to see there is a control mechanism inside the EGR valve? That mechanism is the common failure on these trucks. It has to modulate the opening of the valve.

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 it stays open the EGR valve has an internal problem.

Look carefully at EGR number, there is a number 17087196 which is common.

[ChevyTech]

Do you have one open port on the EGR solenoid?

On a negative backpressure EGR solenoid there is one port which is the vent and should be left open but people find a spot they think should have a vacuum hose, so they plug the vent port. The vent port does not have barbs on it like the one that is supposed to go to the EGR valve.

[TrueChevyTech]

Yeah the numbers I posted are right. I quadruple checked
Posted via Mobile Device

[TrueChevyTech]

Yes I also replaced the solenoid, it wasn't holding vacuum very well, vent has filter, I squeezed egr then hooked vacuum gauge, it held at 5 inches (hardest I could squeeze) it also didn't bleed off on start and after warm up
Posted via Mobile Device

[TrueChevyTech]

After doing some research the EGR 17087198 would be factory for the 305 motor that was original for that truck. I'm wondering if I should change to a different EGR. This also makes me wonder if the TBI system is for the 305 as well but that would be a different thread

[TrueChevyTech]

update- I know the truck ran great before so I went ahead and ordered ACDELCO Part # 2145536

[ChevyTech]

Yes I also replaced the solenoid, it wasn't holding vacuum very well, vent has filter, I squeezed egr then hooked vacuum gauge, it held at 5 inches (hardest I could squeeze) it also didn't bleed off on start and after warm up.

It did not bleed off, so your test shows a failed EGR valve.

After doing some research the EGR 17087198 would be factory for the 305 motor that was original for that truck. I'm wondering if I should change to a different EGR. This also makes me wonder if the TBI system is for the 305 as well but that would be a different thread.

I have seen the same EGR valves used on different size engines. If the EGR parts are off the 305, you should be able to get by with them.

update- I know the truck ran great before so I went ahead and ordered ACDELCO Part # 2145536

I am glad to see you are using a quality part. These TBI trucks are picky about the EGR operation.

[TrueChevyTech]

Originally Posted by ChevyTech

I am glad to see you are using a quality part. These TBI trucks are picky about the EGR operation.

Yeah, I also found it cheaper then the competition. I'll post status update in 5-8 days. Of course I chose the freebie shipping option.

[ScottH]

Interested in hearing ur results, I have a similar situation with my 87.

[TrueChevyTech]

Yes just got egr installed runs great. Although sometimes when idling it revs up and throws a check engine light now. But it doesn't bother anything. I would do the apply vacuum to egr either with pump or squeeze the egr and connect a vacuum gauge if it holds pressure it passes the test, then, start the engine and it should bleed off if not the internal bleed valve is bad and egr needs replaced. Also thanks Ron and Chevytech.
Posted via Mobile Device

[ChevyTech]

You’re welcome, and glad things are going well.

If the check engine light came on it should have a code stored, and you can retrieve the code without a scan tool.

You can search over on the 1988-1998 board where I have posted information on most every problem a TBI can get.

Rattle my cage with a PM if have a question for me, because I rarely ever come over to this part of the message board.