1987 tbi Blazer almost stalls under load

[4WDKC]

tps voltage at idle is supposed to be between .52 and .60, having it to low when you you go off maybe starving it of fuel or air. When you get your cable look at the IAC position, it is supposed to be low in the count range, like 10% so anything more than 15 indicates the throttle position is to far closed. Then you need to do the tps relearn.

 

[Snoots]

Checked the ground for ECM. It is good and clean at it's connection and shows strong signal at the connector. Checked SES light with a test light to 12 volts and the light did not come on with the key in the on position and the test light did not come on. Must have a short or bulb is not seated. Now to pull the gauge cluster again. Ugh!! You guys have me thinking about vacuum leaks. When I set base idle, IAC run full in and then disconnected, I cannot get the truck to idle below 700 in park with the idle screw turned all the way out and butterflies bottomed out. I would think that it should go much below that. That makes me think I may have a vacuum leak and cannot get it to idle down below 700 in park. On a side note, my oil pressure at the interior gauge shows 15-20 psi even at start up. It will raise a little with throttle but not much.Once warm, it will drop down to about 5. This is a new gauge from LMC that reads 0-60. The sending unit was replaced about a year ago and has always been this way. I am used to my '03 cummins super high oil pressure and my Ford 4.6 DOHC running 100 psi cold under throttle and about 65 psi once hot under throttle.

700 at idle in park is not that bad. Glad you got the SES issue fixed. If you have any plastic lines connecting vacuum, they can crack and you'll never know it. I replaced mine with stainless steel. End of problem. If you have a 3 prong oil pressure sensor pull the connector and look down inside the sensor. It MAY have a vent hole. If so, moisture MAY have gotten inside and the sensor is shot. GM makes a little rubber plug to put in a new one but that won't fix the old one. A dab of RTV will do the same trick. Don't ask me how I know this. Also, don't always rely on stock meters. They are good and close but you can't really compare them to what you see in another vehicle. Take a professional (oil fed) gauge to where your oil sensor goes and check your pressure that way before you go chasing ghosts. Another thing, IAC's can get scum around the pintle and seat causing idle problems. There is a procedure for setting it but I don't recollect where that is.

 

[Frankenchevy]

your oil pressure seems low. i don't know ***** about tbi, but have you run a compression test? could be informative. valve lash adjustment? again, never worked on this era of sbc...

 

[diesel_lv]

Pulled all vacuum lines and pulled a vacuum on them. They all held vacuum. New IAC valve. That was stated in initial post. If pintle on IAC is bottomed n butterflies are just a hair from being bottomed on the throttle body, at 700 rpm. Then there should logically be no way for idle to go lower due to air. It would have to be a fuel or timing issue. At least this makes sense to me. Any feedback on that. Will low oil pressure cause then fuel pump solenoid to stop pumping fuel? Turned crank shafts back, ccw, 5 degrees to check rotor n slack in timing chain again. Both times I've done this, the engine pops, possibly backfire, then idles fine until I put a load on it at low speed.

 

[Frankenchevy]

my oil pressure at the interior gauge shows 15-20 psi even at start up. It will raise a little with throttle but not much.Once warm, it will drop down to about 5. This is a new gauge from LMC that reads 0-60. The sending unit was replaced about a year ago and has always been this way. I am used to my '03 cummins super high oil pressure and my Ford 4.6 DOHC running 100 psi cold under throttle and about 65 psi once hot under throttle.

I maybe looking at something typical for this engine but these pressures would scare me. I have accurate gauges, my truck is 60psi cold, 50ish psi under load and 15-20ish at hot idle.

 

[Frankenchevy]

I’m just thinking out loud, but I’m wondering if your cam is worn or your oil pump needs to be rebuilt.

 

[Snoots]

What's the fuel pressure? The low oil pressure will shut down the relay for the fuel pump.

 

[Frankenchevy]

I just read that a normal small block Chevy should have 10 to 15 psi of oil pressure per 1000 RPMs.

...Maybe Snoots is on to something about that fuel pump relay...

 

[diesel_lv]

Bought a oil pressure tester today. From same engine port, cold was 52psi at idle, hot 20 psi at idle. So it's not the pressure. Either gauge or sending unit but I'll worry about that later. Did a little fishing around and research on the internet, uh oh, general consensus is if you don't know what the timing for an engine is supposed to be, "go to highest idle then back off a little". So I did that and highest idle then backed off is 8 degrees btdc with the distributor advance wire disconnected. I'm going to wait for the engine to cool down so I can reinstall the oil pressure sending unit then take it for a test drive and see what happens. Who knows. Right now I'm just waiting for the aldl cable to get here so I can connect the computer to the truck and data log what's going on when it does its thing. If it runs on tomorrow, then I would guess a slipped harmonic balancer. But right now until I drive it and data log it, its all speculation. I do not know the fuel pressure. I installed a GM TPI fuel pump and a 15 pound regulator spring. The 15 is supposed to give me 13-15 psi. Again, TB has been completely rebuild with an ACDelco kit. I do not like using cheap aftermarket stuff when it comes to sensors, etc. ACDelco lasted years for a reason.

 

[diesel_lv]

Test drive, still does it with the timing advanced. Reset timing to 0 degrees. Then started reading "
Exhaust gas recirculation (egr) system 1989 tbi". Never checked to see if the EGR valve had a weak spring. Never thought about it. So I disconnected the vacuum hose from the EGR valve and plugged the hose. No vacuum to EGR. Took it for a test drive and runs like a champ. Now, it could be a weak spring or the EGR solenoid is getting a signal when its not supposed to and allowing vacuum through. So tomorrow when I'm not exhausted, I'll trouble shoot the signal. Hopefully it's the EGR valve. Bad signal would most likely mean bad ECM. Stay tuned to the adventures.

 

[1987 GMC Jimmy]

Never heard of that one. I imagine that relates to a stuck open valve in symptoms. You can probably test the valve to see how fast the pintle and diaphragm come back when depressed. As far as testing the solenoid, you could backprobe pin A4 on the ECM connector to see if it's giving you the spec voltage.

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[4WDKC]

Did you do a IAC relearn when you replaced it?

 

[diesel_lv]

No, I didn't do a back pin to A4. I pulled a vacuum on the back side of the EGR solenoid to 15". Then turn the key to on and the vacuum stayed at 15". Telling me that the ECM is not sending constant power to the solenoid or that the wires going to the solenoid are shorting someplace. I followed these steps.

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All steps checked out. But when I disconnected the EGR and plugged the vacuum line running to it, problems went away. The IAC will have nothing to do with this, with the IAC closed as far as it can mechanically go, it idles at 700. So the IAC could not possibly make it idle any lower. But yes, I did a relearn when replacing it.

 

[4WDKC]

No, I didn't do a back pin to A4. I pulled a vacuum on the back side of the EGR solenoid to 15". Then turn the key to on and the vacuum stayed at 15". Telling me that the ECM is not sending constant power to the solenoid or that the wires going to the solenoid are shorting someplace. I followed these steps.

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All steps checked out. But when I disconnected the EGR and plugged the vacuum line running to it, problems went away. The IAC will have nothing to do with this, with the IAC closed as far as it can mechanically go, it idles at 700. So the IAC could not possibly make it idle any lower. But yes, I did a relearn when replacing it.

I was thinking maybe the IAC didn't have time to adjust correctly because it was to far from where it needed to be ie it was to far closed when you let off the throttle quickly and the engine didnt get enough air till the IAC compensated.

 

[diesel_lv]

4WDKC, Thank you for the suggestions though. Ya, I've put in 2 IAC valves. Had a Duralast in it from Autozone. A friend who is a retired GM tech said that it was bad and to put in an ACDelco one. I got one and put it in and did all the steps. It still did same thing. So now I have a spare IAC valve in case the ACDelco one goes bad. I will try the pin out of A4 though just for good measure. No one in the area has an ACDelco EGR valve and Chevy has to order one at $130ish. RockAuto has the correct ACDelco one per Chevy parts guy at only $55. Thats only $3 more than the Duralast one from Autozone. So I will order it tomorrow if the A4 pinout works properly.