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Last minute problem, need help bad.

Leper

Leper

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Hey guys,
I'm helping a buddy get his truck ready for our trip tomorrow and its having some engine problems. It is a TBI motor from a 91 Blazer.
Here is what it is doing now..... When you start it, it will run, but the rpm go up and down several times until the motor finally dies.
When we started working on it, it would start and then die. Start then die, over and over again. Has spark, has fuel, timing is correct.
Here is what we have done..... After checking for bad gas, loose grounds, battery voltage, etc we replaced the temp sensor in the water passage in the intake. That got it to run a little better. Checked, and set, the timing. No change. Noticed the egr valve vac line was broken, fixed it, no change.
Swapped....ecu, dist, coil, temp sensor. One at a time, then all at once, no change.
Here is the odd thing... If you unplug the vac line, from the map sensor, it will idle fine. It wont rev quick(motor falls on its face), but it will slowly rev all the way. If you put the vac line back on, up and down, up and down, up and down with the idle untill it dies. We tried a different map sensor, and changed its orientation(flipped it upside down) no change. The idle air controll valve is working. The injectors are working. I am at a loss here.
The motor was just replaced with a used engine because it was overheated. But, the last time the old motor was started it dod the same things. The motor has a slightly modified harness, but we couldn't find anything wrong with it.
We have not checked for vac leaks at the intake, but seeing as the old motor did the same thing, and this is a known good motor, I don't see that as the problem.
Does this throttle body have a tps(throttle position sensor)?

Any and all help is appreciated. We worked on this thing for 6 hours tonight, and we are leaving to go wheeling tomorrow afternoon. If there is anything you can think of, let me know.
One more thing.....The motor has not had a O2 sensor on it in years. don't know if its important, just remembered it.

Thanks in advance, I'm counting on the combined smarts of the members here. My trip is in your hands.

Leper
 
I'd run over the intake with a propane torch (not lit of course) checking for vacuum leaks.

Ridiculous to run around without an O2 sensor, but it won't affect cold idle.

If you can, check fuel pressure as well. Bad pump, clogged filter, etc.
 
Yes there is a TPS on it, and there is a way to test it, do a search, I would definitely put an O2 sensor on it, if you have headers maybe look into a 3-wire heated one (AC Delco AFS-74), make sure the spray pattern from the injectors is a nice cone shape, no large drops of fuel. And I second the fuel pump and fuel filter suggestions along with a check for vacuum leaks. (fuel filter first of course so easy and quick to replace.)
 
I was thinking bad pump or clogged filter too. Sounds like you already covered alot of your bases by what you checked.
 
The truck has a O2 sensor simulator and is a trail only buggy. The pump and filter will be replaced this morning. We used starting fluid in a spray can last night, checking for vac leaks.

Thanks guys, anything else?
 
Here's the troubleshooting guide out of the Haynes manual.

Engine lopes while idling or idles erratically

1. VACUUM LEAKS

check intake bolts for tightness, check all vacuum hoses, use soapy water or a length of hose to pinpoint vacuum leaks around the manifold gasket.

2. LEAKING EGR VALVE OR PLUGGED PCV

3. AIR FILTER CLOGGED

4. FUEL PUMP NOT DELIVERING SUFFICIENT FUEL

5. LEAKING HEAD GASKET - perform a compression check

6. TIMING CHAIN WORN

7. CAMSHAFT LOBES WORN

8. VALVE CLEARANCE OUT OF ADJUSTMENT

9. IGNITION TIMING OUT OF ADJUSTMENT

10. IGNITION SYSTEM NOT OPERATING PROPERLY

11. THERMOSTATIC AIR CLEANER NOT OPERATING PROPERLY

12. DIRTY OR CLOGGED INJECTORS
 
Check that the IAC valve passage & plunger arn't covered in crap, and check that your idle speed is high enough.
 
jiminycricket said:
Here's the troubleshooting guide out of the Haynes manual.

Engine lopes while idling or idles erratically

1. VACUUM LEAKS

check intake bolts for tightness, check all vacuum hoses, use soapy water or a length of hose to pinpoint vacuum leaks around the manifold gasket.

2. LEAKING EGR VALVE OR PLUGGED PCV

3. AIR FILTER CLOGGED

4. FUEL PUMP NOT DELIVERING SUFFICIENT FUEL

5. LEAKING HEAD GASKET - perform a compression check

6. TIMING CHAIN WORN

7. CAMSHAFT LOBES WORN

8. VALVE CLEARANCE OUT OF ADJUSTMENT

9. IGNITION TIMING OUT OF ADJUSTMENT

10. IGNITION SYSTEM NOT OPERATING PROPERLY

11. THERMOSTATIC AIR CLEANER NOT OPERATING PROPERLY

12. DIRTY OR CLOGGED INJECTORS
Click to expand...

All checked!!!!!


The fuel pump shoots a huge stream of fuel, but we are going to try another one anyway.

Sierra.....I will check the base idle.

There is a check engine light installed, but there are no codes and it has not come on unless we unplugged something.

Keep them coming guys.
 
FIXED!!!!
There was something wrong in the throttle body. Apparently, both of the throttle bodies we tried. If we pinched off the return line, the truck would run. Real rich, but it would run. The third throttle body we tried made it run fine. I don't know much about the internals of the throttle bodies, but it seems the regulator or whatever diverts the gas to the return line was stuck/bypassing. Too much fuel was being returned, and not enough was going through the injectors. That is why pinching the return line helped, I guess. I would still like to know how/why it happened, but at least it is fixed.

Thanks to all that responded. It really helped. I hope to have some sweet pics next week. Now, all that is left to do is reinstall the original wiring harness,fuel pump, map sensor, ignition system, and everything else that was swapped/altered to locate the problem.

Thanks again.
Leper
 
it's been about 8 years since i've seen the guts of one, but the rochester model 220 tbi is pretty simple, so finding the trouble spot in it shouldn't be too hard. my money says weak fuel pressure regulator or spring and/or rotted regulator diaphragm. either way, the regulator would become the path of least resistance and allow the fuel to escape into the return line instead of keeping the go-juice available for the injectors. and, i assume the o2 simulator you said he had is simply a resistor, so the computer doesn't know anything is wrong and the result is a lean condition. hello high erratic idle.


http://www.goingfaster.com/spo/tbidiagrams2.html
 
vortec said:
it's been about 8 years since i've seen the guts of one, but the rochester model 220 tbi is pretty simple, so finding the trouble spot in it shouldn't be too hard. my money says weak fuel pressure regulator or spring and/or rotted regulator diaphragm. either way, the regulator would become the path of least resistance and allow the fuel to escape into the return line instead of keeping the go-juice available for the injectors. and, i assume the o2 simulator you said he had is simply a resistor, so the computer doesn't know anything is wrong and the result is a lean condition. hello high erratic idle.


http://www.goingfaster.com/spo/tbidiagrams2.html
Click to expand...


and no code......
 
oh yeah, that's true. no code.

if it were me, i'd stick the o2 sensor back on. having the truck adjust its own air/fuel ratio is nice, and obd1 keeps it nice and simple. on some obd2 systems with 378 o2 sensors (ok just 4 or so), it's overkill.
 
This would be a good thread for The Injection Section, congrats on diagnosing it Leper. :waytogo:
 
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