1988 chevy 4.3 smell rich runs lean

russellclan · 05-26-2009, 10:36 PM

pls help a troubled mechanic ...1988 chevy c1500 4.3L TBI automatic. "Truck has been sitting for 3 years ran good when it was parked and now not so much ..." so we hear from the customer. after another shop has put on a fuel pump/sending unit and new fuel filter, they decide its a transmission problem . trany shop rebuilts transsmission . then yet another shop replaces the ECM, intake gaskets, rebuilds TBI, ignition module, and pick up coil, cap, rotor, wires, plugs, etc etc etc.now I get it . this guy is pissed off and is real leary of getting yet another wrong diagnosis..and i am at a loss. the truck smells rich and runs lean. fuel pressure is 15 psi..(2 psi higher than spec)and on a tbi it is sapost to be a constant pressure .(from what i understand)so ya that exsplains the smells rich. but at 1/4 to 1/3 throttle under load it starves for gas and runs bud. i can clamp off the return line to the tank at a rubber junction and the truck runs out GREAT.. but still smell rich .

**octaneman** · 05-27-2009, 06:51 AM

Hi Russelclan

Has anyone checked for fuel leaks ? Check the purge cannister and it's connections, after so many hands that have touched the engine it may have been overlooked. Check for vacuum leaks.Look for open and closed loop operations on the O2 sensor, use DVOM to measure resistance on the computer temperature sensor it may give a signal that the engine is at cold start.

Good luck!

russellclan · 05-27-2009, 11:52 AM

the vaccum is @ 20 hg at idle and no leaks detected ...egr valve test good also. i can be sure beyond doubt that there are no fuel leaks. the O2 sensor shows that it is switching lean and rich .. but the black somke out the pipe says more rich than lean.
but the lack of power/miss/stumble @ 1/3 throttle under load is most disconcerning. if you drive it like u stole it, runs better .. wide open throttle or no throttle and its driveable ..:( but wont go if u drive like a sane man ..

**Raylo** · 05-27-2009, 12:11 PM

Not sure about your exact 4.3 version but some later ones at least had 2 engine temp sensors, 1 for the gage and another for the computer. Like Octaneman said the computer engine temp sensor could be sending a way off signal that could change the fuel to an inappropriate mixture for the ambient conditions and possibly prevent closed loop status. The 4.3s I have worked with have gage sensor on drivers side block and computer sensor near the t-stat housing. Man, with all that has been done to that motor it should hum. Good luck, I hope you get it.

**octaneman** · 05-27-2009, 09:47 PM

So far everything checks out, but the computer temp sensor is not being ruled out just yet. Check the catalytic converter for blockage including the rear muffler see if there is reasonable back pressure coming out of the exhaust. If the exhaust is blocked, isolate the catalytic converter and work your way to the muffler.

post back your findings.

russellclan · 05-27-2009, 10:36 PM

more info ... come to find out its a 88 model truck with a 91 motor and wire harnness/computer. as far as temp snrs the values look right enough for me . computer signal vs actual block temp is acurate.the exhaust has been chopped at one point (no converter and stright pipe to muffler) unknown when that happend. but i am gona rule out the exhaust at this point. the issue seems to get worse the longer its driven .

**octaneman** · 05-27-2009, 11:50 PM

If the harness has been changed, then the computer prom may not be the right one for the truck. Is there a torque converter lock switch ( TCC) on the transmission ? If you got a DVOM with bar graph check the TPS for any dead spots on it.

russellclan · 05-28-2009, 12:15 AM

the tranny is a fresh rebuild w/warrenty from a known good tranny shop. it doesnt even have to be moving , just trying to powerstall it . foot on break and in gear and noticable. but the clamping off of the return line to tank is doing such wonders at the moment . temping to just clamp it off and let this guy go . poor fella . hes been raked over the coals a few times ,i just feel for the guy . but the "can't fix , can't charge " rule is in effect here .not many that go by that anymore. but i guess thats why this guy aint too upset that i have had this thing for over a month now and burnt up a tank of gas .......LOL.. well i will look into the prom #s a place i haven't looked .. thanks

**octaneman** · 05-28-2009, 06:59 AM

From your orginal post you said that it smells rich but runs lean, and by blocking the fuel return you are force feeding it more fuel than it needs. There are only limited number of factors that can have such an adverse effect on how the engine runs lean/rich.

1) Fuel pressure : You mentioned that fuel pressure is 15 psi, 2 psi higher than spec. By blocking the return, you are increasing the pressure at the regulator by a factor of 2, thus giving it the rich smell. The fuel pump has been replaced so it is automatically ruled out. ( unless it's defective)

2) Computer temperature sensor: From the last post, you mentioned that the values on the computer sensor in relation to engine temperature look ok. One way to be sure is by letting the engine get hot then shut off the engine, and using an ohm meter check the resistance at the sensor in relation to engine temperature ( that is if you can get access). Since you are convinced it's ok, that is ruled out also.

Throttle position sensor: If there is a dead spot on the TPS, it may not be noticeable at idle, but under load or driving at highway speed it is very noticeable. Using a DVOM on it should give a clear indication. If you don't have a bar graph DVOM, you can use the tps voltage as a reference, when you move the throttle plate up or down you should see the voltage increase or decrease in steady increments. You give no info on the tps so that is my primary suspect.

Ignition timing: The computer controls the timing from a small wire that is separated from the main harness, ( looks like a fusible link ) by disconnecting it, you should see a noticeable difference in engine speed at idle. Since the computer has been replaced and it was ruled out, but you mention that the engine maybe is a 91, so if the computer is from a 91 and using a prom from a 91 the INTERNAL harness to the computer is an 88 then the rules change and everything is focused on the prom at this point. Unless the truck harness has been completely overhauled from the firewall all the way to the computer, you will need the schematic from both 88 and 91 to look at every pin from point-to-point to be sure.

Oxygen sensor: If the cross counts of open/closed loop from 0.45v to 0.9v and there is no discoloration and the oxygen sensor doesn't appear sluggish, then it's also been ruled out.

If the harness has been touched, look for any splicing of connections or grounds that maybe missing or modified. If you do have a scanner available, re-program the prom to fool the engine that all readings are within spec and see if anything changes.

post your findings on that prom issue.

**octaneman** · 05-28-2009, 08:22 AM

Addendum to my previous post.

One other factor that can cause the lean/rich factor is the Mass Air Flow. The mass airflow sensor frequently causes problems that do not store trouble codes. Moving the throttle with engine running should immediately change the voltage or frequency output.Sensing element contamination can slow MAF sensor response. this can cause stumble during acceleration. A bad mass air flow sensor can cause , stalling, hesitation, surge, poor fuel economy, and sag.

russellclan · 05-29-2009, 11:32 AM

may have found problem :^) hopes are high.. upon close inspection of the wire harrnes from ecm to the engin i have found chaffed,and exsposed wires that look likes they need to be repaired 1 of the 5 wires leading to the fuel pump realy .. probly due to the bad motor mount. but the proof is in the pudding.
i'll know more if it works after i make some wire repairs .thanks

**Raylo** · 05-29-2009, 06:58 PM

Wow, talk about deja vu. My 1994 S-10 had a wire chafing problem, too. It was in the main bundle going through the firewall and every time I shifted the auto tranny from 3 to 4 or back to park a fuse would blow. I lost a few things, including the trany shifting control for tcc lockup and overdrive. But luckily no other driveability issues. Took me awhile to track this one down. It turned out that my problem was a build issue. They simply ran the bundle too close to the shift mechanism and the shifting linkage had an apogee at one point of its rotation and that eventually wore through the protective spiral wrap and then the insulation of one wire and grounded it out every time it passed by. I ended up pulling the bundle away from the shift linkage and tying it off. Not sure if your issue is in the same bundle but there are lots of wires in there. Almost anything bad could happen.

05-26-2009, 10:36 PM	#1 (permalink)
russellclan Registered User Join Date: May 2009 Posts: 11 OS: windows xp	1988 chevy 4.3 smell rich runs lean pls help a troubled mechanic ...1988 chevy c1500 4.3L TBI automatic. "Truck has been sitting for 3 years ran good when it was parked and now not so much ..." so we hear from the customer. after another shop has put on a fuel pump/sending unit and new fuel filter, they decide its a transmission problem . trany shop rebuilts transsmission . then yet another shop replaces the ECM, intake gaskets, rebuilds TBI, ignition module, and pick up coil, cap, rotor, wires, plugs, etc etc etc.now I get it . this guy is pissed off and is real leary of getting yet another wrong diagnosis..and i am at a loss. the truck smells rich and runs lean. fuel pressure is 15 psi..(2 psi higher than spec)and on a tbi it is sapost to be a constant pressure .(from what i understand)so ya that exsplains the smells rich. but at 1/4 to 1/3 throttle under load it starves for gas and runs bud. i can clamp off the return line to the tank at a rubber junction and the truck runs out GREAT.. but still smell rich .

05-27-2009, 06:51 AM	#2 (permalink)
octaneman Mentor Home Support, Gearhead Automotive Center Join Date: May 2009 Posts: 541 OS: WINXP	Re: 1988 chevy 4.3 smell rich runs lean Hi Russelclan Has anyone checked for fuel leaks ? Check the purge cannister and it's connections, after so many hands that have touched the engine it may have been overlooked. Check for vacuum leaks.Look for open and closed loop operations on the O2 sensor, use DVOM to measure resistance on the computer temperature sensor it may give a signal that the engine is at cold start. Good luck! Last edited by octaneman; 05-27-2009 at 07:03 AM.

05-27-2009, 11:52 AM	#3 (permalink)
russellclan Registered User Join Date: May 2009 Posts: 11 OS: windows xp	Re: 1988 chevy 4.3 smell rich runs lean the vaccum is @ 20 hg at idle and no leaks detected ...egr valve test good also. i can be sure beyond doubt that there are no fuel leaks. the O2 sensor shows that it is switching lean and rich .. but the black somke out the pipe says more rich than lean. but the lack of power/miss/stumble @ 1/3 throttle under load is most disconcerning. if you drive it like u stole it, runs better .. wide open throttle or no throttle and its driveable ..:( but wont go if u drive like a sane man ..

05-27-2009, 12:11 PM	#4 (permalink)
Raylo TSF Enthusiast Join Date: Feb 2006 Location: Maryland, USA Posts: 524 OS: XP Desktop, Vista HP Notebook, XP Notebook	Re: 1988 chevy 4.3 smell rich runs lean Not sure about your exact 4.3 version but some later ones at least had 2 engine temp sensors, 1 for the gage and another for the computer. Like Octaneman said the computer engine temp sensor could be sending a way off signal that could change the fuel to an inappropriate mixture for the ambient conditions and possibly prevent closed loop status. The 4.3s I have worked with have gage sensor on drivers side block and computer sensor near the t-stat housing. Man, with all that has been done to that motor it should hum. Good luck, I hope you get it.

05-27-2009, 09:47 PM	#5 (permalink)
octaneman Mentor Home Support, Gearhead Automotive Center Join Date: May 2009 Posts: 541 OS: WINXP	Re: 1988 chevy 4.3 smell rich runs lean So far everything checks out, but the computer temp sensor is not being ruled out just yet. Check the catalytic converter for blockage including the rear muffler see if there is reasonable back pressure coming out of the exhaust. If the exhaust is blocked, isolate the catalytic converter and work your way to the muffler. post back your findings.

05-27-2009, 10:36 PM	#6 (permalink)
russellclan Registered User Join Date: May 2009 Posts: 11 OS: windows xp	Re: 1988 chevy 4.3 smell rich runs lean more info ... come to find out its a 88 model truck with a 91 motor and wire harnness/computer. as far as temp snrs the values look right enough for me . computer signal vs actual block temp is acurate.the exhaust has been chopped at one point (no converter and stright pipe to muffler) unknown when that happend. but i am gona rule out the exhaust at this point. the issue seems to get worse the longer its driven .

05-27-2009, 11:50 PM	#7 (permalink)
octaneman Mentor Home Support, Gearhead Automotive Center Join Date: May 2009 Posts: 541 OS: WINXP	Re: 1988 chevy 4.3 smell rich runs lean If the harness has been changed, then the computer prom may not be the right one for the truck. Is there a torque converter lock switch ( TCC) on the transmission ? If you got a DVOM with bar graph check the TPS for any dead spots on it. Last edited by octaneman; 05-28-2009 at 12:05 AM.

05-28-2009, 12:15 AM	#8 (permalink)
russellclan Registered User Join Date: May 2009 Posts: 11 OS: windows xp	Re: 1988 chevy 4.3 smell rich runs lean the tranny is a fresh rebuild w/warrenty from a known good tranny shop. it doesnt even have to be moving , just trying to powerstall it . foot on break and in gear and noticable. but the clamping off of the return line to tank is doing such wonders at the moment . temping to just clamp it off and let this guy go . poor fella . hes been raked over the coals a few times ,i just feel for the guy . but the "can't fix , can't charge " rule is in effect here .not many that go by that anymore. but i guess thats why this guy aint too upset that i have had this thing for over a month now and burnt up a tank of gas .......LOL.. well i will look into the prom #s a place i haven't looked .. thanks

05-28-2009, 06:59 AM	#9 (permalink)
octaneman Mentor Home Support, Gearhead Automotive Center Join Date: May 2009 Posts: 541 OS: WINXP	Re: 1988 chevy 4.3 smell rich runs lean From your orginal post you said that it smells rich but runs lean, and by blocking the fuel return you are force feeding it more fuel than it needs. There are only limited number of factors that can have such an adverse effect on how the engine runs lean/rich. 1) Fuel pressure : You mentioned that fuel pressure is 15 psi, 2 psi higher than spec. By blocking the return, you are increasing the pressure at the regulator by a factor of 2, thus giving it the rich smell. The fuel pump has been replaced so it is automatically ruled out. ( unless it's defective) 2) Computer temperature sensor: From the last post, you mentioned that the values on the computer sensor in relation to engine temperature look ok. One way to be sure is by letting the engine get hot then shut off the engine, and using an ohm meter check the resistance at the sensor in relation to engine temperature ( that is if you can get access). Since you are convinced it's ok, that is ruled out also. Throttle position sensor: If there is a dead spot on the TPS, it may not be noticeable at idle, but under load or driving at highway speed it is very noticeable. Using a DVOM on it should give a clear indication. If you don't have a bar graph DVOM, you can use the tps voltage as a reference, when you move the throttle plate up or down you should see the voltage increase or decrease in steady increments. You give no info on the tps so that is my primary suspect. Ignition timing: The computer controls the timing from a small wire that is separated from the main harness, ( looks like a fusible link ) by disconnecting it, you should see a noticeable difference in engine speed at idle. Since the computer has been replaced and it was ruled out, but you mention that the engine maybe is a 91, so if the computer is from a 91 and using a prom from a 91 the INTERNAL harness to the computer is an 88 then the rules change and everything is focused on the prom at this point. Unless the truck harness has been completely overhauled from the firewall all the way to the computer, you will need the schematic from both 88 and 91 to look at every pin from point-to-point to be sure. Oxygen sensor: If the cross counts of open/closed loop from 0.45v to 0.9v and there is no discoloration and the oxygen sensor doesn't appear sluggish, then it's also been ruled out. If the harness has been touched, look for any splicing of connections or grounds that maybe missing or modified. If you do have a scanner available, re-program the prom to fool the engine that all readings are within spec and see if anything changes. post your findings on that prom issue. Last edited by octaneman; 05-28-2009 at 07:18 AM.

05-28-2009, 08:22 AM	#10 (permalink)
octaneman Mentor Home Support, Gearhead Automotive Center Join Date: May 2009 Posts: 541 OS: WINXP	Re: 1988 chevy 4.3 smell rich runs lean Addendum to my previous post. One other factor that can cause the lean/rich factor is the Mass Air Flow. The mass airflow sensor frequently causes problems that do not store trouble codes. Moving the throttle with engine running should immediately change the voltage or frequency output.Sensing element contamination can slow MAF sensor response. this can cause stumble during acceleration. A bad mass air flow sensor can cause , stalling, hesitation, surge, poor fuel economy, and sag.

05-29-2009, 11:32 AM	#11 (permalink)
russellclan Registered User Join Date: May 2009 Posts: 11 OS: windows xp	Re: 1988 chevy 4.3 smell rich runs lean may have found problem :^) hopes are high.. upon close inspection of the wire harrnes from ecm to the engin i have found chaffed,and exsposed wires that look likes they need to be repaired 1 of the 5 wires leading to the fuel pump realy .. probly due to the bad motor mount. but the proof is in the pudding. i'll know more if it works after i make some wire repairs .thanks

05-29-2009, 06:58 PM	#12 (permalink)
Raylo TSF Enthusiast Join Date: Feb 2006 Location: Maryland, USA Posts: 524 OS: XP Desktop, Vista HP Notebook, XP Notebook	Re: 1988 chevy 4.3 smell rich runs lean Wow, talk about deja vu. My 1994 S-10 had a wire chafing problem, too. It was in the main bundle going through the firewall and every time I shifted the auto tranny from 3 to 4 or back to park a fuse would blow. I lost a few things, including the trany shifting control for tcc lockup and overdrive. But luckily no other driveability issues. Took me awhile to track this one down. It turned out that my problem was a build issue. They simply ran the bundle too close to the shift mechanism and the shifting linkage had an apogee at one point of its rotation and that eventually wore through the protective spiral wrap and then the insulation of one wire and grounded it out every time it passed by. I ended up pulling the bundle away from the shift linkage and tying it off. Not sure if your issue is in the same bundle but there are lots of wires in there. Almost anything bad could happen.