87 starts and runs, then dies

[Layne02]

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.

 

[1987 GMC Jimmy]

Well, it could be a bad vacuum line, a poor/dirty connection, a wiring break, a defective sensor, or worst case scenario, a bad ECM. I wouldn't even think of the latter until you've ruled out everything else. I'm assuming you've checked the line for cracks and the connector for goop or corrosion so you need to do the ole MAP sensor battery of tests. I've suggested this to people before, and they've been reluctant to do it. I don't know why, though. The only way to be sure if there's a problem with it is to test it.

http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-the-map-sensor-1

 

[Layne02]

I changed vacuum lines, replaced all connectors, nothing gets rid of it. It just runs too good when I first start it for anything to be way off.


Now, I will add this, when I changed both pumps, the one for drivers side from Rockauto quit working after just few days. passenger side pump is an Airtex and seems to be working fine. Don't know if this means anything or not. I just figured cheap pump.

 

[Layne02]

I will do MAP sensor test tomorrow though

 

[1987 GMC Jimmy]

I changed vacuum lines, replaced all connectors, nothing gets rid of it. It just runs too good when I first start it for anything to be way off.


Now, I will add this, when I changed both pumps, the one for drivers side from Rockauto quit working after just few days. passenger side pump is an Airtex and seems to be working fine. Don't know if this means anything or not. I just figured cheap pump.

There's definitely something setting that Code 34 off. Whether it's a problem that's unapparent at this time or a malfunction, there's something up. I wouldn't jump to assume there's something wrong with the the good fuel pump right now. I'd attack the Code 34. Do you know what your vacuum is at idle? Just to make sure your not loosing vacuum (bad EGR valve, base gasket, etc.) or running too low of one.

 

[Layne02]

I'll have to check again tomorrow, but I think I checked it when first got it in and it was at 21 .

 

[1987 GMC Jimmy]

I'll have to check again tomorrow, but I think I checked it when first got it in and it was at 21 .

21's perfect, but if you've done major work something might have changed. You'll get it figured out. Just break out the vacuum gauge and multimeter, get to testing, and you'll be on your way. One of the MAP tests calls for a vacuum pump. I don't know if you have one, but you might be able to borrow one from Autozone if you don't.

 

[Layne02]

Well I meant that the 21 was after the rebuild and getting it started. I have a pump some where around here.

SO if map sensor does test ok, then what direction do I go in?

Did I read correctly that code 34 is low voltage in MAP sensor?

 

[1987 GMC Jimmy]

If the three MAP sensor tests go okay, I'd borrow a fuel pressure tester from Autozone and make sure you're getting 13 pounds, and if that goes well, I'd then assume that the ECM is malfunctioning and needs to be replaced. Oh, just as a housekeeping thing, is your ground strap alright between the firewall and the cylinder head? I've seen the TPS malfunction when that was messed up, and I don't know where the MAP ground circuit ends up, but I thought it was worth mentioning just the same.

 

[Layne02]

I think I have a fuel pressure gauge , just have to see where I can hook it up, has to be rubber hose hook up.

Ground straps should be good. Have one from cylinder block to firewall , and one from frame to block., frame to battery, battery to firewall.

I,ll test tomorrow. Thanks for your help.

 

[1987 GMC Jimmy]

The normal place is to tee it in at the fuel filter. Actron makes a fuel pressure adapter that threads in behind the TBI, and then the steel line threads into the adapter. It stays there all the time, though, and you just have to pop the pressure gauge on when you want to check it.

 

[Layne02]

ok, so tried the map sensor test this morning on the new map sensor I installed when I rebuilt engine, got 5v on no.1 wire from ecm, got good ground on no. 3 wire, BUT, only got 1.1 v on no. 2 wire. It would decrease in vacuum but didn't have good output voltage. Sooo, I decided to test old map sensor, it had good voltage and ground on all, BUT, wouldn't decrease in vacuum. So, is it possible that I have 2 bad map sensors? Could I be that unlucky?

 

[1987 GMC Jimmy]

Okay, it's supposed to be 1.1V at 21" of vacuum. It's supposed to be about 4.7V resting. What vacuum level did you get that 1.1 at?

 

[Layne02]

I did all the testing with key just switched on, motor not started.

 

[Layne02]

I repeated it several times to make sure. The new map sensor was a cheapo off ebay. May need to get the OEM sensor.