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Old 07-20-2007   #1
george88gta
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hard start and intermittant miss

OK, I have just about run out of ideas on this. Car starts hard all the time. By that I mean takes a while to fire up. Turns over fine. Takes about 5-8 seconds of cranking to start it. A little less when warmed up. After it starts, it runs rough for about 45 seconds , smooths out and then it has a slight miss. No particular rpm. I have completely replaced the electrical ignition system, some parts more than once. Numerous sensors were replaced as well. 3 different sets of injectors and a new engine. Had this problem when I first purchased the car (used), engine ate up some bearings so I replaced the original 305 with a GM 300 hp 350. New ECM, custom chip(s). New AC Delco fuel pump and sending unit. New fuel filters. The injector leak down test is good and the cold start valve test is good as well. Running a Holley adjustable regulator set as 44 psi. Had this problem with the GM regulator as well so I doubt that it is the regulator. As an experiment today, I installed one of those neon inline spark testers. I plugged it in to the coil wire right at the distributor cap. Car barely ran when cold. I removed it, warmed up engine reinstalled the tester and car ran rough. Removed it again and it was back to its old normal self. Is this a valid test that is indicating weak spark, or is this not a valid test? Any and all ideas are welcome (except sell the car).
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Old 07-20-2007   #2
MunsonGTA
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Re: hard start and intermittant miss

what is your timing set at? what kind of spark plugs are you using?
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Old 07-20-2007   #3
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Re: hard start and intermittant miss

I am with Munson, where is your timin set at?
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1991 GTA Trans Am
w/305,231 miles
on the original motor!!!!
Time of Death on motor, 4:07pm on 05/29/2010.
Hp:211.2@4200rpms/Tq.328.5@2700rpms
Best et: 13.959@97.05mph
Best mph: 98.3
Best 60' : 1.875 on BFG Comp TA's w/ 32 lbs of air in them
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Old 07-20-2007   #4
nblanchard
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Re: hard start and intermittant miss

Have you replaced the intake temp sensor under the plenum? Also... one of your plugs could be fouled... I have also heard the pcik up coil in the base of the distributor going bad as well. All i can think of for now.
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Old 07-20-2007   #5
george88gta
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Re: hard start and intermittant miss

Quote:
Originally Posted by nblanchard
Have you replaced the intake temp sensor under the plenum? Also... one of your plugs could be fouled... I have also heard the pcik up coil in the base of the distributor going bad as well. All i can think of for now.
Timing is set at 6 degrees advanced. Recommended by chip tuner. I have had the timing as far as 10 degrees advanced, no real difference except that it pings. Plugs are NGK (UR4?) Gapped at .045" as per GM info Less than 500 miles on them. I will have to check my records, but I do not believe that I have changed the sensor under the plenum. Distributor has been replaced in its entirety, no original parts there.
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Old 07-20-2007   #6
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Re: hard start and intermittant miss

Quote:
Originally Posted by nblanchard
Have you replaced the intake temp sensor under the plenum? Also... one of your plugs could be fouled... I have also heard the pcik up coil in the base of the distributor going bad as well. All i can think of for now.
Ever since I relocated my IAT, I've noticed quicker starts, especially when the engine is already warmed up.
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Old 07-20-2007   #7
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Re: hard start and intermittant miss

Quote:
Originally Posted by Stephen
Ever since I relocated my IAT, I've noticed quicker starts, especially when the engine is already warmed up.
How did you relocate it?
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Old 07-20-2007   #8
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Re: hard start and intermittant miss

The stock TPI IAT is merely a coolant temp sensor, so it'll pick up the heat soak from the plenum.

If you get a TBI IAT, it is an actual air temp sensor, that has a filament instead.

The write up article is gone, from my local clubs switch over, in VBB code. I'll try to sum it...

My new IAT is just after my MAF, on the bottom of the plastic intake housing. You can't even see it, when I pop the hood. I got 2 washers that fit the threads, and a nut from Lowes. By sandwiching the washers on the plastic (one inside, one outside) it securely holds the new IAT in place, and the nut secures it. I left the stock IAT in place, instead of just plugging the hole. Now my IAT reads the TRUE incoming air temp, instead of the hot plenum temp.

My mpg, on average, increased by about 2 mpg. Not much, true, And could be seen as a fluctuation. But I keep track of my mpg, and have noticed it stays higher.

Now, for the first tank, after the IAT relocation? My mpg DROPPED down to 9 mpg, from 13 mpg-14 mpg. Now, I'm staying in the HIGH 15s (15.5mpg/15.9mpg) I'm assuming that had to mostly with the ECM tables being redone. And yes, I disconnected my battery, during the relocation, to reset the ECM to learning mode. So the drop in mpg didn't surprise me.
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Old 07-20-2007   #9
george88gta
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Re: hard start and intermittant miss

Quote:
Originally Posted by Stephen
The stock TPI IAT is merely a coolant temp sensor, so it'll pick up the heat soak from the plenum.

If you get a TBI IAT, it is an actual air temp sensor, that has a filament instead.

The write up article is gone, from my local clubs switch over, in VBB code. I'll try to sum it...

My new IAT is just after my MAF, on the bottom of the plastic intake housing. You can't even see it, when I pop the hood. I got 2 washers that fit the threads, and a nut from Lowes. By sandwiching the washers on the plastic (one inside, one outside) it securely holds the new IAT in place, and the nut secures it. I left the stock IAT in place, instead of just plugging the hole. Now my IAT reads the TRUE incoming air temp, instead of the hot plenum temp.

My mpg, on average, increased by about 2 mpg. Not much, true, And could be seen as a fluctuation. But I keep track of my mpg, and have noticed it stays higher.

Now, for the first tank, after the IAT relocation? My mpg DROPPED down to 9 mpg, from 13 mpg-14 mpg. Now, I'm staying in the HIGH 15s (15.5mpg/15.9mpg) I'm assuming that had to mostly with the ECM tables being redone. And yes, I disconnected my battery, during the relocation, to reset the ECM to learning mode. So the drop in mpg didn't surprise me.
Interesting. Did you use a V6 IAT sensor from a 1988 Fbody? I had previously noticed that the stock IAT it is the same as the temp sensor. I can see this helping with warm starts and is certainly worth a try. Dont know what it would do for a cold start issue and the intermittant miss. My mpg is between 16 - 18 mpg right now. If I can get the miss out, I would expect the mpg to go up.
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Old 07-21-2007   #10
george88gta
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Re: hard start and intermittant miss

Quote:
Originally Posted by Stephen
The stock TPI IAT is merely a coolant temp sensor, so it'll pick up the heat soak from the plenum.

If you get a TBI IAT, it is an actual air temp sensor, that has a filament instead.

The write up article is gone, from my local clubs switch over, in VBB code. I'll try to sum it...

My new IAT is just after my MAF, on the bottom of the plastic intake housing. You can't even see it, when I pop the hood. I got 2 washers that fit the threads, and a nut from Lowes. By sandwiching the washers on the plastic (one inside, one outside) it securely holds the new IAT in place, and the nut secures it. I left the stock IAT in place, instead of just plugging the hole. Now my IAT reads the TRUE incoming air temp, instead of the hot plenum temp.

My mpg, on average, increased by about 2 mpg. Not much, true, And could be seen as a fluctuation. But I keep track of my mpg, and have noticed it stays higher.

Now, for the first tank, after the IAT relocation? My mpg DROPPED down to 9 mpg, from 13 mpg-14 mpg. Now, I'm staying in the HIGH 15s (15.5mpg/15.9mpg) I'm assuming that had to mostly with the ECM tables being redone. And yes, I disconnected my battery, during the relocation, to reset the ECM to learning mode. So the drop in mpg didn't surprise me.
New IAT sensor is on its way. Should be here early next week. I will give that a try.
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Old 07-22-2007   #11
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Re: hard start and intermittant miss

Quote:
Originally Posted by nblanchard
Have you replaced the intake temp sensor under the plenum? Also... one of your plugs could be fouled... I have also heard the pcik up coil in the base of the distributor going bad as well. All i can think of for now.
Forgot to answer this one. I replaced the plugs recently. Ones in the engine were the wrong heat range and gap. When I removed them, they were light brown to ashen white in color. If anything, they were indicating running lean. I am also going to remove th eStreet and Performance throttle body spacer. It is difficult to get a good seal on the intake rubber hose. Maybe it is leaking air in to the engine and it would be after the MAF.
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Old 07-22-2007   #12
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Re: hard start and intermittant miss

Well, I am making progress. It barely runs at all. Soon it should quit all together and I will be able to find out what it wants. I removed the Street and Performance spacer, redjusted the TPS and TV cable. Started hard and ran really rough until it warmed up. Then it settled down to a miss. I checked my records and cant find any indication that I replaced the IAT sensor so I will start with that when it arrives. I also have a new AC Delco distributor ignition module on the way. The one in the distributor is an Accel and of all of the ones I have around here, it runs the best with that. Since the module replacement is straight forward, I will do that first. More to follow.
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Old 07-20-2007   #13
nblanchard
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Re: hard start and intermittant miss

yeah... I had a hard starting problem before too... when I polished the plenum and runners, I decided to change that sensor for shits and giggles, since you can't get to it otherwise... and when I got the car back together, never had a hard starting problem again. Does your car burn any oil on start up?
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325.8ft-lbs @ 3250rpm (at the wheels)
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Old 07-20-2007   #14
george88gta
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Re: hard start and intermittant miss

Quote:
Originally Posted by nblanchard
yeah... I had a hard starting problem before too... when I polished the plenum and runners, I decided to change that sensor for shits and giggles, since you can't get to it otherwise... and when I got the car back together, never had a hard starting problem again. Does your car burn any oil on start up?
Sounds like a good place to start. Not looking forward to removing the plenum again, but I am getting pretty good at that task. Recently, I have noticed a small puff of blue smoke if the car sits for a week or so. Valve seals are soon to be required. Damn shame, engine has less than 20,000 miles on it. I have a set of those new style valve seals, a kit from GM. Yet another task I would rather have avoided.
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Old 07-20-2007   #15
FearThisGTA
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GTA Year: 1991
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Re: hard start and intermittant miss

Hmmm, I gotta try that. I wonder where I should put it since mine is a Map car. Any idea's?
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1991 GTA Trans Am
w/305,231 miles
on the original motor!!!!
Time of Death on motor, 4:07pm on 05/29/2010.
Hp:211.2@4200rpms/Tq.328.5@2700rpms
Best et: 13.959@97.05mph
Best mph: 98.3
Best 60' : 1.875 on BFG Comp TA's w/ 32 lbs of air in them
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Old 07-20-2007   #16
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Re: hard start and intermittant miss

Anywhere after the filter. I just put mine after the MAF, because there was a good turn there, putting the IAT right in the airstream, where the most pressure was, and it's hidden. You can even tell it is there, no matter how hard you try. You hafta pull the tubing off, to see it. We even routed the wiring under the TB, and followed the intake tubing, so it is hidden. Of course, wire loom was used too.
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Old 07-21-2007   #17
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Re: hard start and intermittant miss

Quote:
Originally Posted by fearthisgta
Hmmm, I gotta try that. I wonder where I should put it since mine is a Map car. Any idea's?
I'd move it as close to the filter as possible. My Impala's was in the factory airbox itself even with it being a MAF set up.


EDIT- and actually you'd probably want it on the filter side of a MAF car any way, to reduce the chances of getting any unmetered air going into the engine, in case it's not air tight.
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Last edited by Arrow; 07-21-2007 at 11:25 AM.
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Old 07-21-2007   #18
MunsonGTA
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Re: hard start and intermittant miss

CTS could be the problem, i'd look into replacing that.
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Old 07-21-2007   #19
george88gta
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Arrow Re: hard start and intermittant miss

Quote:
Originally Posted by MunsonGTA
CTS could be the problem, i'd look into replacing that.
Coolant temp sensor was one of the first things I tried. No one wants to comment on the inline spark tester? Is that a valid test or does it add too much resistance. One other thing I should mention. The cap and rotor is an MSD set, brass terminals. I have to clean the terminals annually, they get covered with this green corrosion which is very hard. Have to scrape it away.
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Old 07-22-2007   #20
FearThisGTA
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GTA Year: 1991
GTA Engine: 350 TPI Automatic
Re: hard start and intermittant miss

Jet makes a re-location kit and I may look in to gettin that one.
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Cliff Jones
1991 GTA Trans Am
w/305,231 miles
on the original motor!!!!
Time of Death on motor, 4:07pm on 05/29/2010.
Hp:211.2@4200rpms/Tq.328.5@2700rpms
Best et: 13.959@97.05mph
Best mph: 98.3
Best 60' : 1.875 on BFG Comp TA's w/ 32 lbs of air in them
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