87 starts and runs, then dies

[1987 GMC Jimmy]

Oh, I'm sorry. I should have been more specific. I was talking about the voltage relative to vacuum. Did it not descend properly? 1.1 is good when the sensor is pumped up to 21" of vacuum. It should have had around 4.7 with just the key on. If so, it's definitely possible that both were bad. I've had a part go bad, and the replacement was bad several times. ECM and ignition control module come to mind. Electronic parts are usually the worst ones, especially cheap ones. The Delco MAP sensor is the best way to go bar none. I'm sure you can get it for a fair price on RockAuto, and don't forget the five percent discount code found in their preferred vendor section.

 

[Layne02]

Just when you think you get something figured out...........So I picked up another MAP sensor , the $55 dollar one this time. Plugged it in, ran all the tests on this one, everything checked out perfect. Started truck and it ran awful. I mean it was running good and smooth before, would just die out. Now, its stumbling , still wanting to die. And now on occasion I get an intermittent code 33, which seems to be right opposite of code 34. So back to the drawing board.

 

[1987 GMC Jimmy]

It sounds like your wiring and sensor are good. Your vacuum could just be falling, and that's why the 33 is getting tripped. Have you tried running it with the MAP sensor unplugged. If still the same, time to break out the fuel pressure tester.

 

[Layne02]

Yea, unplugged map sensor and couldnt get it to start at all.

 

[Layne02]

I didnt put vacuum gauge on it today but the way it was running I am sure vacuum would be down from normal, like you said, probably what caused the 33.

 

[1987 GMC Jimmy]

Hmm, normally unplugging the MAP sensor would defer input to the TPS sensor, and it would richen up pretty good. You'd get a low voltage code for the TPS if something was wrong with it. Yeah, I think it's time to proceed further and check the fuel pressure.

 

[Layne02]

I will, not sure when I can get to it but thats next step I guess. I just don't get why it suddenly started running so rough after putting a good working MAP sensor on.

Oh well, I'll keep pecking at it.

 

[smoothandlow84]

Ok, so I rebuilt the 87's 305 a while back and have it in, replaced everything on it. Also replaced fuel tanks , lines and pumps. Now, it will fire off and run fine for about a minute, then suddenly just dies. I've watched injectors squirting fuel fine right up til it dies, so seems to be getting gas fine. I do get a code 34, so I put old MAP sensor back on, same thing happens and still get the code 34. What else could make it get code 34? I have checked for vacuum leaks and can not find any. Also, the code CEL pops on after about 10 seconds, even after being reset. Any ideas? Thanks everyone.

Also, it doesn't stumble and die, just dies suddenly, like gas gets cut off.

Replace the IAC sensor.

 

[Layne02]

Well I changed it when I rebuilt engine, I think I even put old one back in at one point when I was trying to figure out problem. I will test it though , just to see if I can cross it off the list . This thing has me baffled.

 

[1987 GMC Jimmy]

I think if you check the fuel pressure, and that passes, you can look to the ECM or maybe a fruity ignition control module. The IAC is worth double checking, but I don't know if that's gonna lead anywhere. Just my $0.02.

 

[Layne02]

Well I have learned a lesson, do not buy the cheap aftermarket parts. I was thinking about the cheap MAP sensor being bad, maybe other parts are not working properly. Put the 30 year old original coil back on, truck ran a lot better. Soo, decided going to change all parts and sensors back out AGAIN with ACDELCO parts. Got a new coil, ignition module, and IAC . Truck runs a whole lot better. Still think IAC has not gotten adjusted to ECM yet but its a lot better. Vacuum gauge stays around 21-22. It will drop a degree every now and then. Truck doesn't die in gear anymore either. So, over next few weeks, I plan on replacing TPS and EGR back out with ACDELCO parts. No more cheap parts for me. Thanks to all.

 

[1987 GMC Jimmy]

Go ahead and do an IAC relearn. Turn the key on, jump the A and B pins for thirty seconds, disconnect the IAC connector, go back and turn the key off, undo the jumper from the ALDL, replug the IACV, and start her up.

 

[Layne02]

I will give it a shot tomorrow , thanks

 

[Layne02]

Man do I like working on a carbureted engine. Ok, so an update on this problem. Had truck running really well, then started acting up, getting an intermittent code 34 again. Took new MAP sensor back, got another, truck running great. Got a new ACDELCO TPS since hadn't changed it out. Truck ran perfectly , went out next day, started it up, ran great, then died. Finally got it started, barely ran. Got another code 34. Now, won't hardly start , but when it does, no code, but won't stay running. What might be causing the MAP sensor to constantly code?

 

[Layne02]

Another day another update. Went out today and started truck up, ran just fine . Idled good, stumbled one time, picked right back up, pulled it around yard and driveway some( no insurance ot tag yet so didnt get on road) , it behaved fine. I dont get it. Will it act up everytime you replace one of the sensors?