87 tbi 350 trouble

[bbchevy]

Over the past few weeks my truck has been getting progressively worse. the idle is around 400, and it has NO power what so ever. had a new fuel filter about 4 months ago alont with cap rotor and wired. plugs a year ago. has a new o2 sensor etc. which sensor would make a tbi motor lose all its power? im not very good with tbi sensors and stuff..... and help would be greatly appreciated

-Scott

[Jonboy]

I would look at the TPS first. Also, make sure your timing is set correctly.

[ChevyTech]

Have you checked it for trouble codes yet?
Do you need instructions on reading the codes without a scan tool?

Something could be causing the knock sensor to determine it is pinging and the timing to get retarded. An engine knock or a loose component on the outside of the engine can cause a high knock count and retarded timing.

Low fuel pressure is another possibility.

[bbchevy]

yea i ran a scan on it the other day. no codes and everything seemed to be within the limits. it only seems to act up under a load

[ChevyTech]

Is this only a light throttle issue?
Does it have problems at heavy and full throttle?
Does it feel like you are trying to pull a train, does it have a flat spot, or does it cut out and almost kill?

[bbchevy]

It does feel like im trying to pull a train actually. just it feels like the train is in reverse and im going nowhere haha. light throttle it seems ok. but the more you give it the more it bogs. but sometimes it will feel ok and it will start to get up to speed. so i will hold the same throttle.... then it will lose it and start to bog then smooth out again... its kinda hard to explain.

[HotRod73]

Maybe Bad Injectors? Or Clogged?

[bbchevy]

just got back from takin a cruise..... my idle quality is now zero... as in it doesnt idle anymore. hmmmm. its hard to take off on a hill with 3 pedals and two feet and a truck that doesnt idle. interesting. guess im going to have to get a ride to work tomarrow. what about a fuel pump? is there any way to test the fuel pressure on tbi motors? and what would i need to do it? im getting more and more lost by the second.

[bbchevy]

this may sound crude.... but i think it might just be time to put "operation carburation" into effect. at least if i go back in time i can diagnose my problems better.....


-Scott

[67mbsc]

I had a similar problem once,on my 87 it drove me nutzzer!!! No codes, nuttin,......... turned out to be the coolant sensor that screws into the gooseneck.
Good luck

[ChevyTech]

Originally Posted by bbchevy

.... what about a fuel pump? is there any way to test the fuel pressure on tbi motors? and what would i need to do it? im getting more and more lost by the second.

TBI systems do not have a fuel pressure test port.

The fuel pressure is tested by using fittings to tee in a pressure gauge. I like to tee in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If pinching off the return line makes a low pressure reading surge above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.


Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.


I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.

http://www.sjdiscounttools.com/ta37650.html

http://www.actron.com/product_detail.php?pid=16174

http://www.actron.com/product_detail.php?pid=16175

http://www.cfm-tech.com/catalog/fuel...er_2940807.htm

[ChevyTech]

light throttle it seems ok. but the more you give it the more it bogs. but sometimes it will feel ok and it will start to get up to speed. so i will hold the same throttle.... then it will lose it and start to bog then smooth out again... its kinda hard to explain.

This also sounds like poor fuel supply could be the problem. Jonboy’s Idea of a bad TPS is also a possibility. A bad TPS can cause symptoms, like the system does not know the throttle is opening so it lacks the fuel needed. TPS trouble codes do not set easily on this system.

Originally Posted by 67mbsc

I had a similar problem once,on my 87 it drove me nutzzer!!! No codes, nuttin,......... turned out to be the coolant sensor that screws into the gooseneck.

I can give you specifications if you want to test the coolant sensor that 67mbsc is referring to.

Do you have an digital ohm meter for testing parts?

[piecesparts]

Look at the MAP sensor for the centeer of your troubles, along with the Throttle position sensor.

[bbchevy]

chevytech- i have an ohm meter. if you wouldnt mind giving me the specs for testing the coolant sensor, that would be great

piecesparts- Is there any way to check the map and tps? unlike the coolant sensor, the map and tps are a little more pricey. so i would rather not change them if they arent broken yet.

[GCISLANDBOY]

I might have an option for ya. i have an 88 with 350 tbi. had the motor rebuilt 2 yrs ago. bout every 3-4 months i would lose power something like what you do. it runs fine if you keep the rpms up, but when you roll on the gas at low rpm, it stutters really bad. and sometimes wont idle. found that the number 8 cylinder was losing compression. many mechanics couldnt figure out why. ended up finding that my cam had rounded itself off. was just the adjustment that i did everytime that kept it going. i would hate if it was that cause it was quite expensive, but might be a possibility.

[ChevyTech]

chevytech- i have an ohm meter. if you wouldnt mind giving me the specs for testing the coolant sensor, that would be great

To test the coolant sensor, unplug the wire connector and test the resistance across the two terminals of the SENSOR with an ohm meter.

Coolant sensor approximate resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.



As for TPS throttle position sensor testing:
Piercing the wires to do testing can cause future problems.
I unplug the sensor, and use jumper wires between the sensor and connector, when I do testing, which is easy when you have wire terminals like the factory uses. (Save old parts to take the terminals out of them)
Test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
It should be about .5 volt at idle and increase smoothly as you slowly open the throttle to about 4 volts at wide open throttle.


Do you have a hand vacuum pump for testing the MAP sensor?
A normal Map sensor reading on the wire that connects to terminal “B” (possibly Lt green) would be 1 to 1.5 volts at idle.
With no vacuum to the Map the voltage on terminal "b" should go over 4 volts.

[ChevyTech]

NOTE: When testing the MAP sensor -

If the engine is running poorly do not expect a normal MAP sensor reading or voltage. If it idles slow, poorly, or rough the vacuum will be lower which will make the voltage higher – out of the normal range.