More expert opinions needed - drivability problemo
11-29-2001, 04:16 PM
#1
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Thread Starter
More expert opinions needed - drivability problemo
Hate to do 'internet' troubleshooting, but you guys are experienced in these motors.......bio - TPI406 motor in my K5 with 24 lb injectors/custom chip/52mm throttle body.
Just put new cap/rotor/coil/MAT (cured my cold start problem, had the scanner tell me it was 175* air on a 36 degree morning - scanner paid for itself already). With these changes it runs perfect cold and has more power than I have ever felt.
Current woe - after driving it for 30 minutes or so around town, get this symptom - after taking off from a light, or after a slowdown, it seems to not really bog, but seems to run low on gas.........it is remedied by stepping harder on the gas pedal and it seems to clear up. This is intermittent so can't pin it down. Does it only when warm and after at least a 20 minute drive. When it does this funky thing and I look at the vacuum gauge I have mounted inside, the vacuum is EXTREMELY low (like 5 inches).
Could it be -
1. Brake booster bad - releasing the brakes causes vacuum problem?
2. MAP sensor
3. TPS - intermittent 'flat spot' when hot?
4. O2 sensor (mounted in header collector - Hedmans)
5. Fuel starvation? Do NOT currently have a fuel pressure gauge so don't know where I am at cold versus warm. Why would it only do it randomly and not under full throttle conditions? BTW, have a Bosch inline just in front of the fuel tank with new filter.
Other areas to focus on with my scanner? I have the new Auto Xray so can get any parameter you guys think I should focus on.
Also, is the O2 sensor supposed to fluctuate from the 700 range down to the double digits (70-90) during its 'normal' course of events. I knew they bounce but that far?
Any help or directions to look I appreciate.
------------------
'79K5 Blazer with 406TPI
[This message has been edited by BlazinOR (edited November 29, 2001).]
Just put new cap/rotor/coil/MAT (cured my cold start problem, had the scanner tell me it was 175* air on a 36 degree morning - scanner paid for itself already). With these changes it runs perfect cold and has more power than I have ever felt.
Current woe - after driving it for 30 minutes or so around town, get this symptom - after taking off from a light, or after a slowdown, it seems to not really bog, but seems to run low on gas.........it is remedied by stepping harder on the gas pedal and it seems to clear up. This is intermittent so can't pin it down. Does it only when warm and after at least a 20 minute drive. When it does this funky thing and I look at the vacuum gauge I have mounted inside, the vacuum is EXTREMELY low (like 5 inches).
Could it be -
1. Brake booster bad - releasing the brakes causes vacuum problem?
2. MAP sensor
3. TPS - intermittent 'flat spot' when hot?
4. O2 sensor (mounted in header collector - Hedmans)
5. Fuel starvation? Do NOT currently have a fuel pressure gauge so don't know where I am at cold versus warm. Why would it only do it randomly and not under full throttle conditions? BTW, have a Bosch inline just in front of the fuel tank with new filter.
Other areas to focus on with my scanner? I have the new Auto Xray so can get any parameter you guys think I should focus on.
Also, is the O2 sensor supposed to fluctuate from the 700 range down to the double digits (70-90) during its 'normal' course of events. I knew they bounce but that far?
Any help or directions to look I appreciate.
------------------
'79K5 Blazer with 406TPI
[This message has been edited by BlazinOR (edited November 29, 2001).]
11-29-2001, 04:25 PM
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I'm sure I'm not qualified to answer, but...
Naturally vacume will be low when you're "bogging" im not familiar enough with vacum readings however to know what 'normal' is. Have you ran a fuel pressure gauge and tapped it to your window and drive around a while? with that many cubes you're gunna need alot of pressure. Good luck!
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Naturally vacume will be low when you're "bogging" im not familiar enough with vacum readings however to know what 'normal' is. Have you ran a fuel pressure gauge and tapped it to your window and drive around a while? with that many cubes you're gunna need alot of pressure. Good luck!
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11-30-2001, 11:17 AM
#3
Member
Thread Starter
Good point..........you pointed out the obvious that I didn't see.......lol.
Anyway, does this sound like injector(s) leaking?
------------------
'79K5 Blazer with 406TPI
Anyway, does this sound like injector(s) leaking?
------------------
'79K5 Blazer with 406TPI
11-30-2001, 12:05 PM
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If you have leaky injectors you'll get some smoke at startup. I've been there 
Also if its increadibly bad and you've got a ring problem you'll get some gassy oil
I live in Oregon too! one of the few but proud
Also if its increadibly bad and you've got a ring problem you'll get some gassy oil
I live in Oregon too! one of the few but proud
11-30-2001, 12:17 PM
#5
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Thread Starter
The motor is just rebuilt, so is extremely tight. Thought I read somewhere that injectors show their 'going bad' when warm, and are fine when cold..........any thoughts to this?
Love it here.........of course this rain is getting old.
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'79K5 Blazer with 406TPI
Love it here.........of course this rain is getting old.
------------------
'79K5 Blazer with 406TPI
11-30-2001, 12:19 PM
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No doubt, im a few hours south of you and we just got through with a huge rain storm thats about to start again tomorrow.
yea it would make sense for injectors to show signs when warm because the coils in the solinoid would heat up and touch causing shorts etc. Thats a possiblity, also it seems to me for the CI you're runing you should be on bigger injectors than 24, i was gunna run 26 on a 383. Depends on HP though.
(take any plug cuts yet? seen any lean/rich plugs)
[This message has been edited by tpi_roc (edited November 30, 2001).]
yea it would make sense for injectors to show signs when warm because the coils in the solinoid would heat up and touch causing shorts etc. Thats a possiblity, also it seems to me for the CI you're runing you should be on bigger injectors than 24, i was gunna run 26 on a 383. Depends on HP though.
(take any plug cuts yet? seen any lean/rich plugs)
[This message has been edited by tpi_roc (edited November 30, 2001).]
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12-02-2001, 10:34 PM
#8
Supreme Member
O2 sensor fluctating is normal, what's the O2 reading during the problem?
If you think the O2 sensor has an issue mounted where it is, then it'd be sending the ECM into open loop...so monitor for open/closed loop during the problem.
If you suspect fuel starvation (I don't think so..but) then O2 should show lean and stay lean..but FP gauge would be the best. most accuarate way to monitor. Fuel pumps would be weaker the longer they ran, but much easier/cheaper to monitor and confirm/deny than just replace the pump to see. Then again, I've no experience running an inline pump though the stock one.
Intermittant flat spots in the TPS? hmmn, not likely but could be easlily seen on your scanner, huh?
MAP will/should change in relation to the intake vacuum you're monitoring.
If you think the O2 sensor has an issue mounted where it is, then it'd be sending the ECM into open loop...so monitor for open/closed loop during the problem.
If you suspect fuel starvation (I don't think so..but) then O2 should show lean and stay lean..but FP gauge would be the best. most accuarate way to monitor. Fuel pumps would be weaker the longer they ran, but much easier/cheaper to monitor and confirm/deny than just replace the pump to see. Then again, I've no experience running an inline pump though the stock one.
Intermittant flat spots in the TPS? hmmn, not likely but could be easlily seen on your scanner, huh?
MAP will/should change in relation to the intake vacuum you're monitoring.
12-03-2001, 12:53 AM
#9
Hot Potato (or Blazin Oregon - however you like it),
Don't forget about the EGR system. A sticky or sluggish valve could still be partially open after leaving a "cruise mode" and returning to idle or part throttle. That can also cause a very low vacuum reading, like the 5" you are reporting. Opening the throttle farther would drop manifold vacuum even more, tending to force the valve shut via the return spring.
Of course, this is only one possibility, but it wasn't mentioned earlier. And, of course, it doesn't enter into the equation if you don't have an EGR system.
You've covered the other causes pretty well. Since you have an AutoXray at your disposal, does the MAP agree with the dash instrument when the condition occurs?
The TPS can be tested with a DMM or the Xray on a hot engine after it is shut off. Turn the ignition "ON" and range the throttle from stop to stop while measuring. Any "dead" spots should be apparent, and should also show up on the injector pulse width timings on the Xray when running.
You can perform a crude test on the brake booster by measuring idle vacuum, then disconnecting and capping the vacuum line, restart, and retest vacuum. Any change in readings could indicate a leaking booster. I had that once on a truck and it made none of the typical leaking noise in the passenger compartment at all. It was a PITA to diagnose, too.
Since you have no apparent fuel starvation problems when opening the throttles further, I would doubt that it is related to fuel delivery. The only variable in that system is the pressure regulator. If you suspect that component, disconnect and cap the vacuum line to the FPR and drive it that way for a couple of days. Since the vacuum to the FPR wouldn't be modulating, you could eliminate that variable. I keep a rubber vacuum cap stuck in a spare hole in the right upper strut mount to bypass the FPR vacuum at will. (Increased fuel pressure comes in handy once in a while.)
Don't condemn the injectors just yet. Excessively leaking injectors would tend to enrich the mixture, which might not typically cause the problems you are describing without some other, more obvious symptoms. It would take a LOT of leakage to stall the engine at part-throttle.
And yes, diagnosing via the web can be a challenge...
------------------
Later,
Vader
------------------
If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader
Don't forget about the EGR system. A sticky or sluggish valve could still be partially open after leaving a "cruise mode" and returning to idle or part throttle. That can also cause a very low vacuum reading, like the 5" you are reporting. Opening the throttle farther would drop manifold vacuum even more, tending to force the valve shut via the return spring.
Of course, this is only one possibility, but it wasn't mentioned earlier. And, of course, it doesn't enter into the equation if you don't have an EGR system.
You've covered the other causes pretty well. Since you have an AutoXray at your disposal, does the MAP agree with the dash instrument when the condition occurs?
The TPS can be tested with a DMM or the Xray on a hot engine after it is shut off. Turn the ignition "ON" and range the throttle from stop to stop while measuring. Any "dead" spots should be apparent, and should also show up on the injector pulse width timings on the Xray when running.
You can perform a crude test on the brake booster by measuring idle vacuum, then disconnecting and capping the vacuum line, restart, and retest vacuum. Any change in readings could indicate a leaking booster. I had that once on a truck and it made none of the typical leaking noise in the passenger compartment at all. It was a PITA to diagnose, too.
Since you have no apparent fuel starvation problems when opening the throttles further, I would doubt that it is related to fuel delivery. The only variable in that system is the pressure regulator. If you suspect that component, disconnect and cap the vacuum line to the FPR and drive it that way for a couple of days. Since the vacuum to the FPR wouldn't be modulating, you could eliminate that variable. I keep a rubber vacuum cap stuck in a spare hole in the right upper strut mount to bypass the FPR vacuum at will. (Increased fuel pressure comes in handy once in a while.)
Don't condemn the injectors just yet. Excessively leaking injectors would tend to enrich the mixture, which might not typically cause the problems you are describing without some other, more obvious symptoms. It would take a LOT of leakage to stall the engine at part-throttle.
And yes, diagnosing via the web can be a challenge...
------------------
Later,
Vader
------------------
If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader
12-03-2001, 06:20 PM
#10
Member
Thread Starter
<font face="Verdana, Arial" size="2">Originally posted by 8Mike9:
O2 sensor fluctating is normal, what's the O2 reading during the problem?
If you think the O2 sensor has an issue mounted where it is, then it'd be sending the ECM into open loop...so monitor for open/closed loop during the problem.
If you suspect fuel starvation (I don't think so..but) then O2 should show lean and stay lean..but FP gauge would be the best. most accuarate way to monitor. Fuel pumps would be weaker the longer they ran, but much easier/cheaper to monitor and confirm/deny than just replace the pump to see. Then again, I've no experience running an inline pump though the stock one.
Intermittant flat spots in the TPS? hmmn, not likely but could be easlily seen on your scanner, huh?
MAP will/should change in relation to the intake vacuum you're monitoring.</font>
O2 sensor fluctating is normal, what's the O2 reading during the problem?
If you think the O2 sensor has an issue mounted where it is, then it'd be sending the ECM into open loop...so monitor for open/closed loop during the problem.
If you suspect fuel starvation (I don't think so..but) then O2 should show lean and stay lean..but FP gauge would be the best. most accuarate way to monitor. Fuel pumps would be weaker the longer they ran, but much easier/cheaper to monitor and confirm/deny than just replace the pump to see. Then again, I've no experience running an inline pump though the stock one.
Intermittant flat spots in the TPS? hmmn, not likely but could be easlily seen on your scanner, huh?
MAP will/should change in relation to the intake vacuum you're monitoring.</font>
Thanks for the feedback..........the O2 bounces from 100-650ish, and as you know, changes rapidly. Just am a newbie
at this EFI stuff and want to learn if what I see is normal.Fuel starvation I am believing is NOT the case either........it continues to go rich/lean/rich/lean even during this symptom, so think you're right about that.
It does not go back and forth from closed to open loop, so O2 is probably OK.
EGR is looking more and more like the culprit...........can I simply take the vacuum hose off, plug it to disable the EGR for a quick test?
------------------
'79K5 Blazer with 406TPI
12-03-2001, 06:23 PM
#11
Member
Thread Starter
<font face="Verdana, Arial" size="2">Originally posted by Vader:
Hot Potato (or Blazin Oregon - however you like it),
Don't forget about the EGR system. A sticky or sluggish valve could still be partially open after leaving a "cruise mode" and returning to idle or part throttle. That can also cause a very low vacuum reading, like the 5" you are reporting. Opening the throttle farther would drop manifold vacuum even more, tending to force the valve shut via the return spring.
Of course, this is only one possibility, but it wasn't mentioned earlier. And, of course, it doesn't enter into the equation if you don't have an EGR system.
You've covered the other causes pretty well. Since you have an AutoXray at your disposal, does the MAP agree with the dash instrument when the condition occurs?
The TPS can be tested with a DMM or the Xray on a hot engine after it is shut off. Turn the ignition "ON" and range the throttle from stop to stop while measuring. Any "dead" spots should be apparent, and should also show up on the injector pulse width timings on the Xray when running.
You can perform a crude test on the brake booster by measuring idle vacuum, then disconnecting and capping the vacuum line, restart, and retest vacuum. Any change in readings could indicate a leaking booster. I had that once on a truck and it made none of the typical leaking noise in the passenger compartment at all. It was a PITA to diagnose, too.
Since you have no apparent fuel starvation problems when opening the throttles further, I would doubt that it is related to fuel delivery. The only variable in that system is the pressure regulator. If you suspect that component, disconnect and cap the vacuum line to the FPR and drive it that way for a couple of days. Since the vacuum to the FPR wouldn't be modulating, you could eliminate that variable. I keep a rubber vacuum cap stuck in a spare hole in the right upper strut mount to bypass the FPR vacuum at will. (Increased fuel pressure comes in handy once in a while.)
Don't condemn the injectors just yet. Excessively leaking injectors would tend to enrich the mixture, which might not typically cause the problems you are describing without some other, more obvious symptoms. It would take a LOT of leakage to stall the engine at part-throttle.
And yes, diagnosing via the web can be a challenge...
</font>
Hot Potato (or Blazin Oregon - however you like it),
Don't forget about the EGR system. A sticky or sluggish valve could still be partially open after leaving a "cruise mode" and returning to idle or part throttle. That can also cause a very low vacuum reading, like the 5" you are reporting. Opening the throttle farther would drop manifold vacuum even more, tending to force the valve shut via the return spring.
Of course, this is only one possibility, but it wasn't mentioned earlier. And, of course, it doesn't enter into the equation if you don't have an EGR system.
You've covered the other causes pretty well. Since you have an AutoXray at your disposal, does the MAP agree with the dash instrument when the condition occurs?
The TPS can be tested with a DMM or the Xray on a hot engine after it is shut off. Turn the ignition "ON" and range the throttle from stop to stop while measuring. Any "dead" spots should be apparent, and should also show up on the injector pulse width timings on the Xray when running.
You can perform a crude test on the brake booster by measuring idle vacuum, then disconnecting and capping the vacuum line, restart, and retest vacuum. Any change in readings could indicate a leaking booster. I had that once on a truck and it made none of the typical leaking noise in the passenger compartment at all. It was a PITA to diagnose, too.
Since you have no apparent fuel starvation problems when opening the throttles further, I would doubt that it is related to fuel delivery. The only variable in that system is the pressure regulator. If you suspect that component, disconnect and cap the vacuum line to the FPR and drive it that way for a couple of days. Since the vacuum to the FPR wouldn't be modulating, you could eliminate that variable. I keep a rubber vacuum cap stuck in a spare hole in the right upper strut mount to bypass the FPR vacuum at will. (Increased fuel pressure comes in handy once in a while.)
Don't condemn the injectors just yet. Excessively leaking injectors would tend to enrich the mixture, which might not typically cause the problems you are describing without some other, more obvious symptoms. It would take a LOT of leakage to stall the engine at part-throttle.
And yes, diagnosing via the web can be a challenge...
</font>
Seems the EGR is my next focus area......can I simply remove its vacuum hose (I think it is fed by a vac hose, right? to test it? Or is there another way to disable the EGR?
Again, thanks.
------------------
'79K5 Blazer with 406TPI
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