VERY odd problem.... help?
Thread Starter
Advanced
Joined: Jul 2010
Posts: 78
Likes: 1
From: Cape Town
VERY odd problem.... help?
I had quite a bit of work done on my 95 LT1 , nothing major but a lot of small stuff... including aircon compressor rebuild (car has done only 40k miles)
Car is now amazing right now except for one problem.
Idle in gear is fine , until you switch on something that draws big electrical power like the aircon , the engines cooling fans , bright lights , heater blower.
The idle then sags , hiccups , revs rise and fall and U can see the display dim a bit as it hiccups , turning the electrical draw off stops this.
Alternator and batt are 100% and the car throws no codes , higher end higher rev performance seems not to be affected , no misses on WOT or anything...
New irridium plugs , plug wires still good , no vacuum leaks , No opti problems ... TPS 100% . IAC reset etc...
Anyone have any suggestions ?
Car is now amazing right now except for one problem.
Idle in gear is fine , until you switch on something that draws big electrical power like the aircon , the engines cooling fans , bright lights , heater blower.
The idle then sags , hiccups , revs rise and fall and U can see the display dim a bit as it hiccups , turning the electrical draw off stops this.
Alternator and batt are 100% and the car throws no codes , higher end higher rev performance seems not to be affected , no misses on WOT or anything...
New irridium plugs , plug wires still good , no vacuum leaks , No opti problems ... TPS 100% . IAC reset etc...
Anyone have any suggestions ?
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
Have your alternator tested. You said it is 100%, but your symptoms say otherwise.
One thing that can happen with alternators is that (depending on the regulator module) sometimes they won't put out enough voltage at low speed (idle). You might also have a problem with one of the diodes in the alternator. The tests done by your average auto parts store on alternators are pretty crude so they might not find these problems.
If you can find somebody who actually rebuilds them it would be beneficial to have a friendly chat with him...
One thing that can happen with alternators is that (depending on the regulator module) sometimes they won't put out enough voltage at low speed (idle). You might also have a problem with one of the diodes in the alternator. The tests done by your average auto parts store on alternators are pretty crude so they might not find these problems.
If you can find somebody who actually rebuilds them it would be beneficial to have a friendly chat with him...
Advanced
Joined: May 2014
Posts: 56
Likes: 0
From: Lake Forest Park WA
So, initially sounds like a possible ground connection has been disturbed ...... or undisturbed could be corrosion resistance showed its head when the added load of the magnet to the compressor joined the draw.
First I would check for codes. Next a DVOM (or Volt meter) check system Voltage with key off (SB 12.5-13.1V), then with engine running and NO ACCESSORIES (S/B 13.5-15.1 V). These Vary, but you get the Idea is "does the alternator even make any charge."
If yes, then we load test. Preferably a real load tester - puts a measured load on the battery while measuring current from the alternator.
Alternators are rated, usually stated on the unit somewhere, yours like a 150 amp or something, anyway you put that much load on the battery, your inductive meter should read your alternator putting out that full rating, if it does NOT, take it off to your parts house and have them load test it, replace if it can not put out it's rating in amps.
This is a common idle issue source.
So, say the alternator tests strong. - Next test the battery.
Usually able to use the same load tester.... AND BE SURE BATTERY IS FULLY CHARGED.
Typically the load is 1/2 of the batteries cold cranking amp rating, so if your battery is 700 CCA, you will put a 350 amp draw on the battery, for 15 to 30 seconds the Voltage should not drop below 9.5 Volts after 30 seconds. Then after load is let up, watch for voltage recovery. You should see the battery recover to a minimum of 12.5V within a minute. If the battery Voltage drops below 9.5, - OR - does not recover to at least 12.5, or withing one minute, the battery is BAD.
This also is a common idle issue source.
These MUST be clarified before wasting your time on further diagnostics.
So if the above all pass, then .......
Voltage meter readings at battery engine running no accessories, again should be 13.5-15.1 V, now start to turn on each accessory at a time, what your looking for is one, or possibly combo, that have excessive voltage drop at the battery when they are turned on indicates high current draw.
I have 2 cars that dropped dead at stop lights and hunted for the idle, Mustang was a bad battery, Firebird was high draw cooling fan ( it could barely spin actually).
Also:
So I always consider ALL items done since when worked correct last. Sometimes like it or not a new condition can be result of repair side effect.
First I would check for codes. Next a DVOM (or Volt meter) check system Voltage with key off (SB 12.5-13.1V), then with engine running and NO ACCESSORIES (S/B 13.5-15.1 V). These Vary, but you get the Idea is "does the alternator even make any charge."
If yes, then we load test. Preferably a real load tester - puts a measured load on the battery while measuring current from the alternator.
Alternators are rated, usually stated on the unit somewhere, yours like a 150 amp or something, anyway you put that much load on the battery, your inductive meter should read your alternator putting out that full rating, if it does NOT, take it off to your parts house and have them load test it, replace if it can not put out it's rating in amps.
This is a common idle issue source.
So, say the alternator tests strong. - Next test the battery.
Usually able to use the same load tester.... AND BE SURE BATTERY IS FULLY CHARGED.
Typically the load is 1/2 of the batteries cold cranking amp rating, so if your battery is 700 CCA, you will put a 350 amp draw on the battery, for 15 to 30 seconds the Voltage should not drop below 9.5 Volts after 30 seconds. Then after load is let up, watch for voltage recovery. You should see the battery recover to a minimum of 12.5V within a minute. If the battery Voltage drops below 9.5, - OR - does not recover to at least 12.5, or withing one minute, the battery is BAD.
This also is a common idle issue source.
These MUST be clarified before wasting your time on further diagnostics.
So if the above all pass, then .......
Voltage meter readings at battery engine running no accessories, again should be 13.5-15.1 V, now start to turn on each accessory at a time, what your looking for is one, or possibly combo, that have excessive voltage drop at the battery when they are turned on indicates high current draw.
I have 2 cars that dropped dead at stop lights and hunted for the idle, Mustang was a bad battery, Firebird was high draw cooling fan ( it could barely spin actually).
Also:
So I always consider ALL items done since when worked correct last. Sometimes like it or not a new condition can be result of repair side effect.
Advanced
Joined: May 2014
Posts: 56
Likes: 0
From: Lake Forest Park WA
Oh Yea, I forgot to mention that low voltage WILL cause IDLE HUNT on all types fuel injection. Also, when you scan for codes and there is one or more codes, obviously follow that lead. BUT if there is no code(s), you should be able to find a system voltage line in your data stream, study all your data stream, fuel trims, learns, rpm signals, MAF readings, MAP readings ETC, if they all seem normal while the stutter/hunt is occurring, CHECK THAT VOLTAGE READING. There is a lot more further diagnosis but i would start there.
