5.7 TBI runs rich, hesitates...

[JCampbell]

88’ K20 Suburban. 5.7 TBI with 125,000 miles on it. It starts fine, but idles with a very slight roughness. Smells like it’s running a bit rich. From idle, when I advance the throttle slightly, the engine will begin to choke and die, but if I continue to advance the throttle it will pull out of it and run smooth again. (This only occurs after a short warm up. When cold , it will advance smoothly) I can bypass this by advancing the throttle quickly, though I sometimes notice hesitation. When the engine is fully warm, it will occasionally die while idling or after being shifted into reverse. It has always started right back up. There is an inconsistent miss at idle speed that sometimes appears at higher RPM.

Any thoughts on this?

[rfmaster]

Originally Posted by JCampbell

88’ K20 Suburban. 5.7 TBI with 125,000 miles on it. It starts fine, but idles with a very slight roughness. Smells like it’s running a bit rich. From idle, when I advance the throttle slightly, the engine will begin to choke and die, but if I continue to advance the throttle it will pull out of it and run smooth again. (This only occurs after a short warm up. When cold , it will advance smoothly) I can bypass this by advancing the throttle quickly, though I sometimes notice hesitation. When the engine is fully warm, it will occasionally die while idling or after being shifted into reverse. It has always started right back up. There is an inconsistent miss at idle speed that sometimes appears at higher RPM.

Any thoughts on this?

Check TPS sensor for a worn spot - it happens with older, high millage TPS units. This check can be done without removing TPS from TB (two torx screws). Measure resistance between pins A & B or C & B. While moving throttle from idle position (engine is off) resistance should change smoothly without any abrupt jumps! You should see (depending on pin pair) resistance change from about 1000 Ohms at idle to about 5 to 6 kOhm at WOT (the values maybe reversed depending on the pin pair).

Another common problem is a vacuum leak between TB base and intake manifold. If TB was never removed or serviced before the chances are that mounting gasket is leaking.

Rich condition could be caused by a faulty CTS - also a very common failure point in higher millage TBI's. To check, disconnect connector from TPS (screwed into intake manifold just left of the thermostat housing and should have a two wire connector). With cold engine (say coolant is around 65 to 70F) CTS should read about 2.7 to 3.0 kOhm. A hot CTS ~ coolant around 180F should result in CTS resistance reading about 180 to 200 Ohm.

Inconsistent miss at idle could be due to spark misfire. Check your ignition system - distributor, wires, plugs. A low compression cylinder my also cause an idle miss, but this is a rarity on FI motors.

Questions

Engine performance under load? Acceleration - is it acceptable??

What is your SES light doing during engine start-up??

Do you know how to pull DTC??

Did any one replaced fuel pump before??

//RF

[JCampbell]

I’ll check the TPS and CTS…I’ve heard about the CTS causing it to run rich before, but it must’ve slipped my mind. Thanks.

It does okay under load/acceleration. It’s on 35’s w/ stock gears so it’s a little sluggish, but nothing out of the ordinary. The SES operates normally at startup, but I have noticed that in certain conditions of hard acceleration it will light up, then either go out after a few minutes, or stay on until I shut the truck off - then it goes back to normal operation.

DTC?

*I* never replaced the fuel pump, but I’m not sure if it was ever replaced before I got it.

[rfmaster]

Originally Posted by JCampbell

I’ll check the TPS and CTS…I’ve heard about the CTS causing it to run rich before, but it must’ve slipped my mind. Thanks.

It does okay under load/acceleration. It’s on 35’s w/ stock gears so it’s a little sluggish, but nothing out of the ordinary. The SES operates normally at startup, but I have noticed that in certain conditions of hard acceleration it will light up, then either go out after a few minutes, or stay on until I shut the truck off - then it goes back to normal operation.

DTC?

*I* never replaced the fuel pump, but I’m not sure if it was ever replaced before I got it.

DTC = Diagnostic trouble code.

The 88 model year ECM has OBD-I system - primitive by today's standards, but in its simplicity easy to work with. To pull DTC's insert a jumper between pins A & B of the ALDL connector as shown below:



The ALDL connector can be found just left of the steering column, on the under dash panel:




Insert jumper, turn ignition on, but do not start engine - the SES light should blink 12 12 12 three times (blink, pause, blink, blink, longer pause) followed by any set DTC's that ECM might have pending. This should help with troubleshooting.

//RF

[JCampbell]

Replaced the CTS and it's not running rich anymore. Still hesitates at off idle (especially in reverse :/ ) but I think that *may* be an ERG issue...planning on temporarily bypassing to diagnose.

However, now it "pops". Hard to describe, but it usually happens under acceleration (although it has happened once of twice while motoring down the road) the truck will "pop" and lose power for a split second. Sometimes it just happens once under acceleration, but there are times when it will go through a quick series of "pops" and the truck will bog down and almost die. It's like the truck is starving for fuel for a split second. Weird. I'm stumped.

You mentioned the fuel pump. You think that could be an issue?

[rfmaster]

Re check your base timing with EST connector set open - you may want to advance timing from 0 deg to about 4 deg - see how she likes it. Also, not forget to reset ECM to clear code 43 when done.

//RF

[JCampbell]

Originally Posted by rfmaster

Re check your base timing with EST connector set open

Uhhhh...what?

[rfmaster]

TBI system controls spark plug timing. The ECM depending on operating conditions (and calibration) can alter effective spark plug timing over 40 degrees. On TBI equipped trucks there is no vacuum advance.
ECM does not know if the base spark timing was set correctly. To set base spark plug timing - warm the engine let idle. Locate EST set connector - it is usually taped to the main harness next to the brake booster. It is a single circuit connector and it should look something like this:



The wire can be tan/white or brown/white - color of the wire varied depending on a sub-contractor that made harness for GM. Get your trusty timing light on verify that base timing with EST connector open is 0 deg - engine should be idling around 600-650 RPM. If your engine is suffering from a stretched timing chain (OE timing chains stretch with millage) bump timing to about 2 to 4 deg BTDC - it may help engine performance. Any further advance may cause lean pops at cruise speeds.

When you disconnect the EST connector this will cause ECM to set a fault code - SES light will be lit. In this instance it is a false fault. Shut engine off after verifying and adjusting timing if necessary. Secure dizzy clamp when all said and done. Re-connect EST connector. To clear this fault - disconnect battery for about 30 sec. Done.

With EST connector closed ECM is controlling timing - at idle timing will be bouncing between 16 to 20 deg. Do not try adjusting timing with EST closed.

//RF

[JCampbell]

Roger that. Thanks!

[lurch115]

not to jump in the middle here, but have you pulled the IAC valve to see if its carboned up?

[JCampbell]

'Bout ready to haul this thing off the the #$% @#$% scrap yard!

Reset IAC. Verified IAC plunder was in "closed" position in the TB (meaning I could see the IAC plunder cone by looking down into the throttle body). Unplugged IAC to ensure plunger would not move. Fired engine. Engine went to (what seemed like) full throttle. While the engine was running, I verified the IAC plunger was still in the "closed" position...it was.

I am absolutely out of ideas.

[JCampbell]

Nope. Where's that little gem located at?

[rfmaster]

Originally Posted by JCampbell

Nope. Where's that little gem located at?

Starboard side of the TB, screwed into the TB



//RF

[franky69c10]

Noob ? What's the CTS and where is it located
Posted via Mobile Device

[Jonboy]

Originally Posted by franky69c10

Noob ? What's the CTS and where is it located
Posted via Mobile Device

Coolant Temp Sensor. It's usually located in the front coolant passage on the passengers side of the intake manifold.

[SCOTI]

Good info here. Anyone have a link that has a GM TBI set-up w/reference descriptions of all the various connections? I'm an 'arrow pointed to the part/location w/a description' kind of dinosaur.

[Jonboy]

I don't know a link, but I can get most of them and their locations, at least for a small block truck.

CTS- Coolant temp sensor- right front of intake, in coolant passage, 2 pins in connector
TPS- Throttle Position Sensor- right side of throttle body, 3 pins
INJ- (2)- fuel injectors- top of throttle body. 2 pins each
IAC- Idle Air Control- right side of throttle body, 4 pins
EGR- Exhaust Gas Recirculation request solenoid- right side of intake, near center 2 bolts- 2 pins and vac. line to EGR valve
ESC- Electronic Spark control- on bracket at right side of intake, near center- 5 pins
MAP- Manifold Absolute Pressure sensor- on bracket with ESC, 3 pins, green connector. Will have 1 vac. line to throttle Body
KS- Knock Sensor- in water jacket above pan rail, usually on RH side. 1 pin, large white connector
O2-Oxygen Sensor- usually located in LH exhaust manifold- 1 pin
VSS- Vehicle Speed Sensor- will be on transmission or will use light sensor in buffer at Speedometer head- 2 or 3 pins, depending on type used. Later vehicles use the VSS on the trans, and no speedometer cable (90-91 V, 91R)
Fuel pump relay- located on firewall next to A/C box-4 or 5 pins
Air Switching Solenoid- on AIR hoses, 2 connectors with 2 pins each
ECM- Electronic Control Module- located behind glove box
Vapor canister solenoid- located on top of vapor canister on radiator support- 2 pins
EST- Electronic Spark Timing- 1 pin, in harness near A/C box.
P/N switch- in Park/Neutral switch at base of column, 1 pin

I think that is it. Hope this helps.

[JCampbell]

Okay...checked the timing...about 10 deg out, but I also noticed that the fuel coming out of fuel injectors seemed to be more akin to water spraying out of a shower head, rather than a mist. Is it safe to assume that the ijectors could use a cleaning?

How would I go about cleaining them? Seafoam, or manually?

[rfmaster]

There are couple of ways to clean injectors, but the best all around approach requires fuel pod removal and dis-assembly. To clean injectors and to remove years of crud and fuel additive deposits requires services of fuel injector cleaning service. Over the years I've used Witchhunter performance in Oregon to service my injectors:

http://www.witchhunter.com/

Now - it's possible that you have low fuel pressure - stock TBI is specked between 9 and 13 PSI. TBI will run as low as 6 PSI but engine will stumble and fuel spray will look like shower stream from a clogged shower head. Do you know if fuel pump was ever replaced in your truck - you have electric (in tank) fuel pump.

//RF

[JCampbell]

Fuel pump was never replaced as far as I know.

[rfmaster]

Guess what - time to check fuel pressure.

//RF

[JCampbell]

Yeah, I found this link last night, http://www.howstuffinmycarworks.com/...pressure.htmlI , so I ordered a fuel pressure tester and the little adapter for the GM TBI from Napa this morning. I’ll give her a whirl tonight and let y’all know what I come up with.

For reference:
Fuel Pressure Tester: https://www.napaonline.com/Catalog/C...282460889&An=0
GM TBI Adapter: https://www.napaonline.com/Catalog/C...282483662&An=0

[Jonboy]

I have a gauge that I installed in the inlet side permanently. It came with the engine parts I got, so I'm not sure where it came from. Seems like it is a Mr. Gasket part.

[rfmaster]

"The average fuel pressure for this systems is between 12 psi, and no more than 15 psi." Actually, I seen people run as low as 9 PSI - broken FPR spring would cause flaky fuel pressure.

I have to give it to the guy - nice write-up with photos!!

Use the double wrench technique - this prevents fuel line twist and cracked fuel pod housing.


//RF

[JCampbell]

Hmmm...12 psi, but blocking the return line bumps the line pressure up to about 14-15 psi.

Fuel pressure reg?