Same problem new motor..

[TrueChevyTech]

87 tbi 350
Engine idles fine hot or cold, but when warmed up if i ever so slightly push the gas the engine will stumble, although i can accelerate right out of it without hesitation. this was the main reason I spent a load of cash and had it rebuilt.. either way the motor was wore pretty bad so no wasted dollars there.
I get error 33 out of my check engine light which refers to map sensor i've been researching test methods although it seems I don't have a friend with a vacuum pump, any other ideas?
also the oil pressure gauge goes way down to 15 psi at idle.. i'm going to hook up a manual gauge to test it and just use the stock one as a reference .. that sounds like the correct first approach to me.

[79k30dually]

Assuming you reused the original sensors, verify your oil pressure. I'd look at the 5 volt reference and ground circuits on the map and tps. On your data list you can verify your vacuum to the map with a vacuum gauge. Distributor condition? New fuel filter? fuel pressure ok? I could help more with a data list print. I'll try to help, Ron

[TrueChevyTech]

i'm not sure I know what a data list print is, i'll have to get a vacuum pump but even harbor freight is $40 for one, I could just buy a new MAP sensor

[79k30dually]

Data list print is just the information from the scan tool data list. Do you have a scan tool? A basic vacuum gage will do to test the map. Some auto parts stores have a tool loan program. Ron

[TrueChevyTech]

As far as a scan tool, I have a paper clip and barely know how to use it.I dont have even just a standard vacuum gauge. I'll buy some tools but we need to control ourselves, I'm the one that needs controlled I mean.. As far as some more info. most sensors are from old motor, except the ecm water temp sensor and the oil pressure sender for the gauge.

[79k30dually]

How does the brake pedal feel? The code 33 is for high voltage/low vacuum, vacuum leak is a possibility. Carefully spray carb cleaner or penetrating oil arround the base of the throttle body, while the engine is running, listen for a change in the idle. These engine were also known for splitting the elbow on the front of the TB to the pcv valve. Does the idle speed seem high? Let me know and we will go from there. Ron

[TrueChevyTech]

I cleaned the IAC tonight and now it doesn't idle at all guess I broke it, will have to replace it
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[BigDan3131]

Did you have the throttle body rebuilt or did you not touch it?

[TrueChevyTech]

I didn't touch it although it had these symptoms after sitting for a couple months before i pulled the motor

[79k30dually]

Remove the battery cables and hold both cables together for about 2 minutes. This will discharge the capacitors in the ECM, thus reseting the learned memory. Then start the engine, you'll have to give it some throttle to keep it running. Did you take the plunger out of the motor pack after you removed it?

[TrueChevyTech]

Originally Posted by 79k30dually

Remove the battery cables and hold both cables together for about 2 minutes. This will discharge the capacitors in the ECM, thus reseting the learned memory. Then start the engine, you'll have to give it some throttle to keep it running. Did you take the plunger out of the motor pack after you removed it?

Yup that is a big no no, actually i pulled it out of the throttle body and plugged it in and turned the key on and off and on again, it shot out on it's own .. haha I learned but it cost me $30, also i put the new one in and it idle and started slightly better then (random thought) I walked to the EGR and pulled the vacuum line off. my stumbling problem lessened a bit, so i then unplugged the EGR solenoid and it is even better. I'm not so sure this will pin point anything. I had some throttle body cleaner and made an attempt the spray around the TB gasket, the front in particular and didn't notice a change. I will have to do the battery cable trick tomorrow, I'm also very curious of my fuel pressure

[79k30dually]

How did you set the timing, at what?

[TrueChevyTech]

I am going to get a fuel filter , and also would like to know what fitting and what test gauge to use/buy cheap like harbor freight preferred, if I got just the fitting I could just use an anhydrous ammonia gauge from work, it will read down to 0 but not as accurate that low I'm assuming. Also would the fact that I have 5ft of exhaust on one manifold and 6ft with a glass pack on the other matter?
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[TrueChevyTech]

how does this make you feel lol
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[TrueChevyTech]

The fuel pressure gauge worked I got a reading of 12.5 psi although I didn't get a chance of letting it warm up, I used vacuum gauge on egr and it held. Replaced fuel filter and tested throttle body side of egr solenoid and it didn't hold pressure also when stumbling slightly above idle the egr is completely open is this normal? Also engine ran fine for a minute or two after new filter was installed, so confused. Injector pattern looks good idk what's left..... Fuel pressure regulator spring possibly? Read allot of Chevytechs posts on similar symptoms... Btw I am not him and am not jacking his name. I'm truetech on various computer related forums and just threw Chevy in since name was taken . thanks for dealing with my babbling
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[79k30dually]

Fuel pressure should be 9-13, so thats ok. The EGR should not be opening without the truck moving. Disconnect the vacuum line to the EGR solenoid and plug it, Test for the stumble again, it won't hurt any thing to go on a test drive with it disconnected. Sounds like the EGR solenoid is bad. To test it with everything hooked up, connect the vacuum gage to the line at the EGR valve, You should have 0 vacuum at idle and off idle without the truck moving. Ron

[TrueChevyTech]

Yeah slightly above idle the egr opens all the way, does the gear indicator on the column matter? I'll put a vacuum gauge in line from egr and solenoid tonight
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[79k30dually]

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

[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]

the speedometer works, it's about 5mph off at 60mph is all

[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.

[TrueChevyTech]

Replaced solenoid for egr and it still stumbles either egr is bad or not enough back pressure from exhaust, next step will be exhaust since it will need it anyways, I'm thinking double glass packs each side I'll be buying the pipe and fittings and also a cheap wire welder to get it all put together, it's the cheapest route I can think of
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[TrueChevyTech]

Got a little update, exhaust is on and sounds great, did test and the egr is opening itself slightly above idle , there isn't vacuum coming from solenoid while parked at all next step will be to attempt to clean the egr, any suggested method?
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[79k30dually]

Carb cleaner and and small brush, Ron

[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
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