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98 LS Lugging when accelerated

Old 06-13-17, 08:41 PM
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LS9698
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Default 98 LS Lugging when accelerated

My 98 LS 164k always run like new. Now its acting up. I would appreciate some feedback. Has anyone had this happen, and what caused it.

Symptom
Engine idles smooth, cruises at low and high speed smooth. (Good gas mileage)
However, acceleration at any speed causes it to hesitate and what we called in the carb days "bog down" until an effortless cruise is established.
NO CEL ,no old codes stored.

Current state of maintenance
VVTI valves are new and working, sparkplugs are 7 month's old Denso IK20 Ir. Pulled them, look clean with the exception of one that is very slightly off color from the rest.

Coils are dry w/no carbon scoring, dielectric added to their contacts and harness connections.
Injectors are all clicking away nicely.
All vacuum lines replaced, pcv, and its grommet and hose new.

In an act of desperation I pulled every harness connector and hit 'em with contact cleaner and dielectric.
MAF got cleaning while I was at it.
That is what I've done so far.

I spoke to my trusted mechanic of 15 years. He gives me two possibilities, 1: Fuel pump/filter, 2: Low fuel press under acceleration.

I then went to an "Import Specialist", and told him everything as I'm describing to you guys ^^^^.
His direction in the prognosis is aimed at the ignition system ie: maybe a bad coil.
He does have a Toyota Tech Tool.

Thanks
Comments and Ideas welcome.
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Old 06-13-17, 10:11 PM
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I'd simply try bypassing the fuel pump resistor short circuiting the +B and Fp at the diag. connector.
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Old 06-14-17, 06:52 AM
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I'm beginning to think I have a bad/going bad coil.

Does anyone know how to isolate which one it is?
Toyota Tech stream would be how, but I don't have that.
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Old 06-14-17, 11:34 PM
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Originally Posted by Yamae View Post
I'd simply try bypassing the fuel pump resistor short circuiting the +B and Fp at the diag. connector.
+1 sounds like a fuel pressure issue. Good 1st step advice here. You need to make sure the fuel pump is switching to 'full speed' under load. Normally it runs at half speed until you put your foot in it. Bypassing the resistor puts it at 'full speed' at all times which will rule out relay/ecm issues with regard to the fuel pump.

A bad coil would cause a misfire condition which the ECM should pick up pretty easily and turn on the check engine light.

Last edited by djamps; 06-14-17 at 11:39 PM.
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Old 06-15-17, 03:47 AM
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Originally Posted by djamps View Post
+1 sounds like a fuel pressure issue. Good 1st step advice here. You need to make sure the fuel pump is switching to 'full speed' under load. Normally it runs at half speed until you put your foot in it. Bypassing the resistor puts it at 'full speed' at all times which will rule out relay/ecm issues with regard to the fuel pump.

A bad coil would cause a misfire condition which the ECM should pick up pretty easily and turn on the check engine light.
You are right, djamps. There are feed back signals called IGf1 to IGf8. These are generated by igniters each when the coils are driven. The ECU is monitoring those 8 signals checking phases and levels. When one of them or more is out of the range, the ECU judges that a misfire is occurred and stores the code and turns on the check engine light. On the other hand, the ECU is not monitoring the fuel pressure. You need to use a fuel pressure gauge if you want to check the pressure. Bypassing the resistor is a quick way to know that the problem is related to insufficient fuel supply or not.
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