causes of start-up hesitation?
05-06-14, 10:34 PM
#1
causes of start-up hesitation?
Has anyone experienced consistent start-up hesitation with their SC? My '94 SC400 usually requires holding the key for an uncomfortably long time before firing up. The battery, fuel filter, and spark plugs/wires are new, but that hasn't solved the problem.
What are other possible culprits? Fuel pump? Starter? A leak (fuel pressure) somewhere in the system?
The car runs great once started, so it seems like it's something to do with priming the system initially.
This old thread is the only one I've found so far on the topic:
https://www.clublexus.com/forums/sc-...ark-plugs.html
ECU control (post #2)? Mine is the same whether it's a cold or warm start.
Edit, here's what I've done thus far in hopes of solving the problem; some of it needed anyway as general maintenance:
- new fuel pressure regulator ... no change
- new vacuum hoses ... seemed to help initially on warm starts, then no change
- tightened screw on top of fuel pulsation damper ... seemed to help initially, then no change
- new spark plugs & wires ... no change
- new caps & rotors ... no change (pass. side coil may be a bit weak)
- new fuel filter ... no change
- used MAF (mine was dead) ... no change, but no CEL!
- IAC Valve cleaning ... no change, but noticed a crack in the housing; waiting on replacement
- fuel pump ECU bypass (jumpered Fp and +b pins) ... seemed to help initially, then no change
- used fuel pump ... no change
- disconnected fuel pressure VSV ... no change
- new coolant temp. sensor and connector (damaged and damaged) ... no change
- checked the box below the air intake hose ... not an issue
Things left to do:
- new upstream O2 sensors
- ECU swap? (no sign of leaking capacitors on mine)
- leaky fuel injectors?
- take it to a shop with full diagnostic equipment...
Figured I'd update it a year+ later with additional thoughts:
O2 sensors and fuel injectors were replaced to no apparent effect. The ECU was damaged, after all, but that fixed other problems not the start-up hesitation. A few new vacuum hoses here and there helped, although mostly with warm start symptoms which are now perfect. Going from memory, the biggest improvement came from finding that the return line hose (at the banjo bolt) below the fuel tank was barely on finger tight. I was probably losing pressure there whenever the car sat.
However, as just added to my manifold porting thread: Upon further reflection, having become deeply familiar with the inside of the manifolds, it seems impossible that the '92-94 SC400 wouldn't have a cold start hesitation. The cold start injector is in the upper plenum, and the regular injectors are at the base of the lower manifold. So, cold starts are going to be delayed (longer cranking) by however long it takes for that air volume to be sucked through the upper and lower runners to reach where regular injection takes place. I've probably been chasing my tail in trying to eliminate the last of the start-up delay.
What are other possible culprits? Fuel pump? Starter? A leak (fuel pressure) somewhere in the system?
The car runs great once started, so it seems like it's something to do with priming the system initially.
This old thread is the only one I've found so far on the topic:
https://www.clublexus.com/forums/sc-...ark-plugs.html
ECU control (post #2)? Mine is the same whether it's a cold or warm start.
Edit, here's what I've done thus far in hopes of solving the problem; some of it needed anyway as general maintenance:
- new fuel pressure regulator ... no change
- new vacuum hoses ... seemed to help initially on warm starts, then no change
- tightened screw on top of fuel pulsation damper ... seemed to help initially, then no change
- new spark plugs & wires ... no change
- new caps & rotors ... no change (pass. side coil may be a bit weak)
- new fuel filter ... no change
- used MAF (mine was dead) ... no change, but no CEL!
- IAC Valve cleaning ... no change, but noticed a crack in the housing; waiting on replacement
- fuel pump ECU bypass (jumpered Fp and +b pins) ... seemed to help initially, then no change
- used fuel pump ... no change
- disconnected fuel pressure VSV ... no change
- new coolant temp. sensor and connector (damaged and damaged) ... no change
- checked the box below the air intake hose ... not an issue
Things left to do:
- new upstream O2 sensors
- ECU swap? (no sign of leaking capacitors on mine)
- leaky fuel injectors?
- take it to a shop with full diagnostic equipment...
Figured I'd update it a year+ later with additional thoughts:
O2 sensors and fuel injectors were replaced to no apparent effect. The ECU was damaged, after all, but that fixed other problems not the start-up hesitation. A few new vacuum hoses here and there helped, although mostly with warm start symptoms which are now perfect. Going from memory, the biggest improvement came from finding that the return line hose (at the banjo bolt) below the fuel tank was barely on finger tight. I was probably losing pressure there whenever the car sat.
However, as just added to my manifold porting thread: Upon further reflection, having become deeply familiar with the inside of the manifolds, it seems impossible that the '92-94 SC400 wouldn't have a cold start hesitation. The cold start injector is in the upper plenum, and the regular injectors are at the base of the lower manifold. So, cold starts are going to be delayed (longer cranking) by however long it takes for that air volume to be sucked through the upper and lower runners to reach where regular injection takes place. I've probably been chasing my tail in trying to eliminate the last of the start-up delay.
Last edited by t2d2; 11-21-15 at 06:18 PM.
05-10-14, 09:10 PM
#2
I've only tested once, so it could be blind luck barring additional testing, but it fired right up for the first time that I can remember, so this solution is highly encouraging:
http://community.cartalk.com/discuss.../hard-starting
I had previously been trying to prime the system by pumping the gas pedal with the key in the "on" position, but that didn't do anything. I had the right idea but the wrong approach. 
So, while I search for the next step in solving the matter, does anyone have any suggestions on troubleshooting the anti-drainback valve? Is it purely a matter of trying a different fuel pump?
http://community.cartalk.com/discuss.../hard-starting
The next time you go to start the engine, turn the ignition switch to run position for two seconds and then turn the ignition switch off. Repeat this a half dozen times and then try starting the engine. If the engine starts right up there's a problem with the fuel pump anti-drainback valve.
So, while I search for the next step in solving the matter, does anyone have any suggestions on troubleshooting the anti-drainback valve? Is it purely a matter of trying a different fuel pump?
05-11-14, 07:06 AM
#4
I've been searching extensively and haven't found an answer to this, so here's as good a place as any to ask: What is the stock fuel pump part # for the '94 SC400? (I'm guessing whatever it is, it's the same across all SCs.) Unless I get a used OEM one for a direct swap, I'd like to have a replacement one on hand before going through the trouble of pulling the old one out to determine the part #...
I know it's a Denso, and parts #950-0109 and #950-0110 seem like good bets. Is #950-0110 just a newer version with backward compatibility? On Denso's parts finder, it shows #951-0003, which seems to match #950-0110 in the various interchange listings. So, is anything in that group going to be a straight drop-in replacement?
(densoautoparts.com may have the worst parts finder interface ever. You have to input your vehicle info before you can select anything, and submitting it clears the vehicle info and takes you to another page that requires selecting basically the same thing and inputting your vehicle info again. At first, I thought it was totally broken.)
Edit: Just as I posted that, I got a response to a classifieds PM that I've got a used fuel pump added to my order, so it's hopefully a non-issue.
I know it's a Denso, and parts #950-0109 and #950-0110 seem like good bets. Is #950-0110 just a newer version with backward compatibility? On Denso's parts finder, it shows #951-0003, which seems to match #950-0110 in the various interchange listings. So, is anything in that group going to be a straight drop-in replacement?
(densoautoparts.com may have the worst parts finder interface ever. You have to input your vehicle info before you can select anything, and submitting it clears the vehicle info and takes you to another page that requires selecting basically the same thing and inputting your vehicle info again. At first, I thought it was totally broken.)
Edit: Just as I posted that, I got a response to a classifieds PM that I've got a used fuel pump added to my order, so it's hopefully a non-issue.
Last edited by t2d2; 05-11-14 at 07:28 AM.
05-22-14, 09:10 PM
#5
A simple test today revealed a possible culprit: Shutting off the engine then starting it again in 5 seconds. Starts right up. That would seem to indicate a faulty fuel pressure regulator.
I'm a bit confused if there is one or two FPRs or whether one of them serves a slightly different purpose, and which one would be to blame. (Edit: one FPR and one fuel pulsation damper.)
The used fuel pump swap didn't solve the problem, so that presumably rules out the faulty anti-drainback valve. It seemed promising at first but returned to normal.
I'm a bit confused if there is one or two FPRs or whether one of them serves a slightly different purpose, and which one would be to blame. (Edit: one FPR and one fuel pulsation damper.)
The used fuel pump swap didn't solve the problem, so that presumably rules out the faulty anti-drainback valve. It seemed promising at first but returned to normal.
Last edited by t2d2; 07-18-14 at 03:24 PM.
07-17-14, 10:48 PM
#6
Updated the first post with all my trial and error failures.
In addition to my last post above, I also noted in recent weeks that if I quickly bumped the starter, waited a second, then turned the key again, it would start instantly every time. That would seem to rule out everything but fuel delivery. Replacing the fuel pressure regulator was my long awaited final straw in that respect, but sadly, it had no effect, either.
I've done all the vacuum and VSV checks and not found anything unusual or out of spec so far.
GeorgeNVA had this to offer in another thread:
That fits with some troubleshooting info I ran across a few weeks back, although they ended up turning up a different culprit:
http://www.askamechanic.info/askamec...t/view/111/47/
In addition to my last post above, I also noted in recent weeks that if I quickly bumped the starter, waited a second, then turned the key again, it would start instantly every time. That would seem to rule out everything but fuel delivery. Replacing the fuel pressure regulator was my long awaited final straw in that respect, but sadly, it had no effect, either.
I've done all the vacuum and VSV checks and not found anything unusual or out of spec so far.
GeorgeNVA had this to offer in another thread:
there's a number of reasons for too long cranking time
One of them being leaky injectors. They may fail in quite weird fashion, holding 3-4bar without single drop, but dripping under lower pressure (so there's always some fuel in intake manifold after leaving the car for some hours). Seriously leaking injectors usually can be detected as negative long term fuel trims at idle (say -15%).
Additionally, the trims affect open loop operation (including start-up) which may explain why the problem returns after some modification
You can shed some light onto your problem by trying to reset ECU before each attempt to start, and by pressurizing fuel system (run fuel pump for a couple seconds before cranking)
One of them being leaky injectors. They may fail in quite weird fashion, holding 3-4bar without single drop, but dripping under lower pressure (so there's always some fuel in intake manifold after leaving the car for some hours). Seriously leaking injectors usually can be detected as negative long term fuel trims at idle (say -15%).
Additionally, the trims affect open loop operation (including start-up) which may explain why the problem returns after some modification
You can shed some light onto your problem by trying to reset ECU before each attempt to start, and by pressurizing fuel system (run fuel pump for a couple seconds before cranking)
http://www.askamechanic.info/askamec...t/view/111/47/
07-17-14, 10:58 PM
#7
Another thing I just remembered, and this may be normal but I've never had reason to test it on another car before: when I open the gas cap shortly after filling the tank (as in, within a gallon or two of full), there's so much pressure that gas actually comes up out of the filler spout.
I've read conflicting reports of whether that's bad or by design, and whether it only relates to vapor lock with carbureted cars or also fuel injected. If it is bad, then I should be looking for a blockage in the tank's vent system.
I've read conflicting reports of whether that's bad or by design, and whether it only relates to vapor lock with carbureted cars or also fuel injected. If it is bad, then I should be looking for a blockage in the tank's vent system.
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07-17-14, 11:13 PM
#8
This is for Dodge trucks, but the symptoms (highly pressurized fuel tank and hard starts) and solution match GeorgeNVA's leaky fuel injector theory:
http://www.dieseltruckresource.com/d...ne-t94773.html
A 5+ hour tear-down isn't what I was hoping for (and won't have time for for a few weeks), but that may be the next thing to try if no one has better suggestions.
http://www.dieseltruckresource.com/d...ne-t94773.html
A 5+ hour tear-down isn't what I was hoping for (and won't have time for for a few weeks), but that may be the next thing to try if no one has better suggestions.
07-18-14, 03:16 PM
#9
Well, file this one under "I'm might skeptical, but..."
Following a suggestion I found elsewhere, pertaining to stalling, vapor lock, and excessive tank pressure, I tried a little experiment today. Before starting the car, I opened the filler cap to release pressure from the tank. It started pretty quick the first time. Drove it a short distance, out and about, then 90 minutes later did the same thing and it started instantly.
I'm probably 3 gallons shy of a full tank at the moment, and the first time I opened the cap today, there was enough pressure for gas to flow out. That's with the front of the car parked uphill and sloped a little bit to the passenger side, which would make the filler spout a lower point than normal. The second start-up was on a flatter spot, with the car pointed slightly downhill, and no gas leakage (but also less pressure built up).
http://www.wikihow.com/Fix-a-Car-That-Stalls
I bumped another thread about tank vent blockage; would love to hear people's ideas for investigating that.
Following a suggestion I found elsewhere, pertaining to stalling, vapor lock, and excessive tank pressure, I tried a little experiment today. Before starting the car, I opened the filler cap to release pressure from the tank. It started pretty quick the first time. Drove it a short distance, out and about, then 90 minutes later did the same thing and it started instantly.
I'm probably 3 gallons shy of a full tank at the moment, and the first time I opened the cap today, there was enough pressure for gas to flow out. That's with the front of the car parked uphill and sloped a little bit to the passenger side, which would make the filler spout a lower point than normal. The second start-up was on a flatter spot, with the car pointed slightly downhill, and no gas leakage (but also less pressure built up).
http://www.wikihow.com/Fix-a-Car-That-Stalls
In rare cases, your car may stall due to what is commonly called a "vapor lock", a situation that occurs when the fuel lines supplying gas to the engine become hot enough to boil the gas in the line turning it into vapor. Fuel pumps are designed to pump liquid not “vapor” and thus fail to provide enough fuel pressure. However, a clogged fuel tank vent system can cause a vacuum in the fuel line that causes fuel to eventually stop reaching the engine. This also is a problem unique to carbureted engines. Fuel injected engines commonly have "closed loop" fuel delivery systems that make this condition unlikely to occur. Try opening your gas cap. If you hear a "whooshing" sound, like when you open a can of coffee, it means the gas tank is not venting properly. Now try starting the car. It should work after a couple tries. This is usually found only in older cars. If you have it, this will probably happen again, so your tank vent system should be checked for blockage. More often than not, the problem can be traced to a cheap replacement locking gas cap. A cheap fix is to drill a small hole in your gas cap to allow air in and prevent the vacuum from forming, but the gas cap should not be left this way.
07-19-14, 12:27 AM
#10
Been redirected here from my post on capacitors replacement thread
As for scan tool, I'm not very familiar with pre-1998 ECUs and USDM-specific features (like OBD2-compliance) unfortunately. But you can see long term fuel trims as specific voltage at particular pins of diagnostic connector (VF1,VF2).
Note that LTFT1,LTFT2 parameters are, in general, dependent on RPM, temp, load
As for scan tool, I'm not very familiar with pre-1998 ECUs and USDM-specific features (like OBD2-compliance) unfortunately. But you can see long term fuel trims as specific voltage at particular pins of diagnostic connector (VF1,VF2).
Note that LTFT1,LTFT2 parameters are, in general, dependent on RPM, temp, load
07-19-14, 11:23 AM
#11
Thanks for stopping by. I know next to nothing about monitoring fuel trims, so that whole post flew right over my head... Are you saying the ECU stores long-term fuel trims and can be read from the diagnostics box? The '94 is OBDI, BTW. Don't know if that matters.
I'm working on diagnosing the EVAP system today to see if that's the source of excessive tank pressure. My current challenge is figuring out which of the fuel lines is the vent from the tank up to the charcoal canister, so I can blow compressed air through it and see if there is a blockage. I tested and cleaned the canister already.
I'm also curious if the tank pressure is also the cause of the start/stall problems I've had a few times when the engine is warm. It makes sense that pressure would be at its highest at that point. If that happens again, I'll definitely try releasing pressure at the filler cap and see if it starts and holds.
I know next to nothing about monitoring fuel trims, so that whole post flew right over my head... Are you saying the ECU stores long-term fuel trims and can be read from the diagnostics box? The '94 is OBDI, BTW. Don't know if that matters.I'm working on diagnosing the EVAP system today to see if that's the source of excessive tank pressure. My current challenge is figuring out which of the fuel lines is the vent from the tank up to the charcoal canister, so I can blow compressed air through it and see if there is a blockage. I tested and cleaned the canister already.
I'm also curious if the tank pressure is also the cause of the start/stall problems I've had a few times when the engine is warm. It makes sense that pressure would be at its highest at that point. If that happens again, I'll definitely try releasing pressure at the filler cap and see if it starts and holds.
07-21-14, 02:19 AM
#14
You can read more on the trims topic here
http://alflash.com.ua/vf1.htm
In OBD1 capable Toyotas, these trims are stored in ECU memory. They can be monitored with a scanner or by simply measuring the VF1/VF2 voltages.
http://alflash.com.ua/vf1.htm
In OBD1 capable Toyotas, these trims are stored in ECU memory. They can be monitored with a scanner or by simply measuring the VF1/VF2 voltages.
07-21-14, 09:31 AM
#15
Thanks. I'm not 100% comfortable with digesting all that material, but it at least gives me a good idea of what you're talking about and how to monitor it. I'm pretty sure I'll screw something up and jumper the wrong pin to ground, so best to take that info to the mechanic and let him decipher it. When it comes to electrical, there's an unspoken rule that many details should be glossed over, and I inevitably guess wrong on each of them!
I've reset my ECU so many times with all this testing, I probably don't have much stored in the way of fuel trims at the moment, anyway.
When it comes to electrical, there's an unspoken rule that many details should be glossed over, and I inevitably guess wrong on each of them!I've reset my ECU so many times with all this testing, I probably don't have much stored in the way of fuel trims at the moment, anyway.
