1987 tbi Blazer almost stalls under load

[diesel_lv]

1987 blazer, 350, tbi, 700r4. New ACDelco fuel pump, fuel filter, rebuilt throttle body w new pressure regulator spring, tps, iac, engine temp sensor, hei distributor, coil, wires n plugs. All ACDelco parts. Starts fine, idles fine n drives fine. But as soon as I go off road in sand at idle n give it fuel, it starts to cut out like it's running out of fuel. Then if I've been driving n pull into a parking lot at idle, it does the same thing. Does not do it all the time in parking lot, but always in the sand under load. Oh, also has new O2 sensor and did it w and w/out the catalytic. Tps reads .49 at closed throttle. Set iac to idle at 700 in park but had to back screw all the way out then just a turn in to keep throttle blades from sticking. Checked timing chain lash, move 5 degrees on timing pointer before distributor starts to move. Is this too much? I'm at wits end. Ordered aldl cable but won't be here for a week. Any suggestions? Thank you in advance

 

[1987 GMC Jimmy]

I would say five degrees is on the high side of the acceptable range. Three is more ideal. You should be able to watch the timing mark behavior with a timing light with the bypass disconnected to see if it holds steady at idle and then if you give it gas and let of, is it stable or wobbly. Did you notice if the baffling in the fuel tank was in good shape? IIRC, the factory baffling was plastic and disintegrates over time, especially if left to sit. I think the better version of this question is will it do this on a full tank of gas? I would check for dead spots in the TPS. Did you use a new base gasket when you rebuilt the TBI? Any vacuum leaks, or is everything good there? That includes the EGR valve being stuck open or not holding vacuum? The best way to test the former is to pull it off and make sure the diaphragm moves and while you're doing that clean the carbon off the pintle. The best way to test the latter would be to plug off the vacuum hose and see if it does better without it. Different valves use pressure in different ways so I don't think there's a one size fits all test for them besides that. I might test the MAP sensor, too, speaking of vacuum. I'll attach a test for you to see if you want to try that. Are you getting your ECM self-test light when you first turn the key on? Any codes?

 

[diesel_lv]

Haven't checked the timing while reving it. BUT at idle it holds steady. Did not check baffling but it does it w tank full or low. Checked tps n it doesn't appear to have any dead spots. Yes, new base gasket. Have found no vacuum leaks. The CEL is intermittent. It will light up when the key is turned to on sometimes. When it does, it goes off after I start it. It will work for a couple days then I don't get it to turn on at all. Thinking might be a short somewhere on that. I am going to pull the EGR valve this afternoon after the engine cools down n clean it. Thank you for the suggestions.

 

[1987 GMC Jimmy]

All of that checks out to me except the SES light. Big red flag there. It maybe a grounding issue or the bulb for the light may not be seated all the way. Have you tried pulling codes yet to see how it acts doing that?

 

[Rusty Nail]

Grounding issue x2

 

[diesel_lv]

No, I have not tried pulling codes from it. I just pulled EGR valve and it holds vacuum just fine. But it lets carb cleaner slowly leak past the shaft. I also did a test of the EGR solenoid. It clicks when power is applied. I tried blowing through both vacuum ports. The port that leads to the EGR valve always allows air to be blown through it but it comes out of the filter port. The end that pulls vacuum from the throttle body will sometimes allow air to be blown through and sometimes it closes all the way. This indicates that the solenoid is intermittent and should be replaced. As for the EGR valve itself, is a slow leak of carb cleaner past the shaft acceptable?

 

[diesel_lv]

Replaced EGR solenoid and still same thing. CEL does not light up when jumping a and b in port under dash. So I will have to do some troubleshooting tomorrow on that. Timing shows that at idle it is 0 degrees when warm. As I rev the engine, it advances but does not jump around. When the distributor advance wire is disconnected it stay at 0 degrees when rev'd. So all of that seems to be working properly. Engine pulls 21" of vacuum at idle. Holds steady. Any other suggestions?

 

[1987 GMC Jimmy]

I think the SES light is everything here. Your ground seems to be the most likely suspect. If your truck hasn’t been hacked up or you haven’t had any rodent activity, I don’t think it’ll be a wiring problem in the harness. Worst case scenario you have a bad ECM which isn’t a big deal, but I wouldn’t bank on that just yet.

 

[diesel_lv]

It doesn't seem to be hacked up. Broken connectors is the engine compartment but everything seems good there. How would I diagnose a bad ECM? It actually stalled tonight on test drive after replacing EGR solenoid while in the sand. Never stalled before, just idled very low. It sounds like it had a radical cam n lopes very low when it does it.

 

[Rusty Nail]

Bad ground.

 

[Honky Kong jr]

Vacuum leaks?

 

[1987 GMC Jimmy]

Vacuum leaks?

I think it's a fallacy to ever discount vacuum leaks as a culprit for anything because they can always pop up. Not so much on a TBI or something with a lot of stuff deleted, but if you still have all that black spaghetti under your hood, anything can happen. He does have 21" of Hg on his gauge at idle, which is really good, though.

As far as diagnosing a bad ECM goes, if your ground checks out good, I've encountered partial failures and complete failures so it can be a little tricky. You'll have a diagnostic circuit quietly fail in one where you can't pull codes or interface with it in any way, but everything else will still work fine, or you'll have EST/ESC circuit(s) fail and the timing will be all out of whack and it'll run like crap, or you'll look over at it and see it smoking. All of these have happened to me unfortunately. You can open it up and see popped circuits and/or smell the board to tell if it's bad sometimes but not always. I've unplugged it completely to get the solid SES light to avoid digging around for the bulb or pulling the cluster and also to make sure that the wiring related to the light is okay. Take care of that ground first, though. It may just need the little bolt tightened up on it or some corrosion cleaned off it, and you'll be on your way. Unless something just fried in the ECM, the fact that it was doing right some of the time tells me that it's likely okay.

 

[diesel_lv]

Thank you 1987 GMC Jimmy. I will check the ground for the ECM and do the test light at 12v positive to the SES light circuit after unplugging the ECM. I've gone through all the vacuum lines. If there is a leak, I have no idea. And yes, it has strong vacuum.

 

[diesel_lv]

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.

 

[diesel_lv]

Got SES light to work. Just wasn't seated fully in socket. Lotta work for that. Checked for codes and all it flashes is 12 so no codes. Sprayed brake parts cleaner all around throttle body to include base and shaft and no rise in rpm.