Replaced spark plugs, Engine almost dies when started

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Capd323

Junior Member
Nov 5, 2011
4
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I have a 1990 lincoln town car, 54000 miles, just replaced the original spark plugs with brand new iridium plugs. It's got a 5.0L V8.

This only happens when it is started cold, after the engine warms up for a minute or so it will run strong. It starts fine but afterwards it cycles from normal revs to extremely low revs, so low that you can tell it is about to cut out. After doing this about 5 times it will be just fine until of course it cools down and i try to start it again.

I put the manufacturer recommended gap on all of them, the wires are all hooked up to the correct cylinder as i installed them one at a time. I can't think of much that would cause this, except maybe the new iridium plugs might need a different gap since the others were 20 years old? I am stumped. Any advice or ideas would be appreciated. thanks
 
foghorn67

foghorn67

Lifer
Jan 3, 2006
11,883
63
91
alfa147x said:
Try cheap plugs
Click to expand...
this. Try getting the OEM Motorcraft or cheap Autolite plugs. Also, look at the terminals of the plug boots, might need cleaning. If you have a cheap voltmeter, switch it to ohms and check resistance for each plug wire.
 
PlasmaBomb

PlasmaBomb

Lifer
Nov 19, 2004
11,636
2
81
How does the manufacturers recommended gap compare to the iridium plug companies recommended gap?

They could be different...
 
pandemonium

pandemonium

Golden Member
Mar 17, 2011
1,777
76
91
I'm guessing there wasn't this problem prior to swapping the plugs...
This may sound silly, but did you try resetting the ECU?
Torque the plugs correctly?
Does it feel like a cylinder is misfiring?
I know Fords have problems with their fuel pumps, and what you describe sounds a lot like that.
I'd also try some fuel injector cleaner and see if that helps.
 
dawp

dawp

Lifer
Jul 2, 2005
11,347
2,709
136
considering the age of the car (1990), I would have done a full tune-up including plugs, wires, cap and rotor, air filter and changed the oil and filter.\

also, is it possible you may have swapped some of the plug wires?
 
FuzzyDunlop

FuzzyDunlop

Diamond Member
Jan 30, 2008
3,260
12
81
It may simply be the fact that they are irridium. my mr2 runs like crap with irridium.
 
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Capd323

Junior Member
Nov 5, 2011
4
0
0
It ran fine before this. It's actually running fine now the only time i have any problem is when i start it cold, the warmer it is outside the less it will almost stall. when i started it for the first time after putting in the new ones it was around 40 degrees and it almost stalled 5 or 6 times. i just now tried starting it and its about 55 degrees outside, it only revved down once. the problme seems to be related to temperature. There are no cylinders missing, i could drive it all day and i don't think it would miss a beat. I'll check out the IAC though and see how it looks.Thnaks for the suggestions
 
Zenmervolt

Zenmervolt

Elite member
Oct 22, 2000
24,514
34
91
Did you double-check the heat range of the new plugs? It's possible that the new plugs are simply too cold for the engine and are acting up on cold starts.

ZV
 
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Capd323

Junior Member
Nov 5, 2011
4
0
0
The IAC is fine, very clean. I don't think it's the problem. I looked at all the hoses i may have bumped when installing them, they're all fine. I'm thinking the problem is simply the spark plugs aren't compatible with my engine.
 
PlasmaBomb

PlasmaBomb

Lifer
Nov 19, 2004
11,636
2
81
Zenmervolt said:
Did you double-check the heat range of the new plugs? It's possible that the new plugs are simply too cold for the engine and are acting up on cold starts.

ZV
Click to expand...

This would have been my next suggestion, followed by buy some copper plugs :D.
 
pcgeek11

pcgeek11

Lifer
Jun 12, 2005
22,100
4,886
136
You screwed the pooch by changing the gap... I would carefully gap them back to where they were and try that. Check to make sure you didn't break off the iridium tip of the electrode.

Denso says:

Q. Do I need to set the gap?
A. Generally, no. The DENSO Iridium Power plug comes pre-set with a protective sleeve over the firing end, to protect the gap from accidental alteration. DENSO Iridium Power’s ultra-efficient firing power compensates for normally recommended gap settings that are smaller than the pre-set value. In the cases of vehicle modifications (nitrous, turbo-chargers, super-chargers, high power ignition systems, etc.), some adjustments may be desired. Or, if you prefer to remain consistent with factory specifications, you may adjust. However, please be careful not to place any stress on the fine center electrode during adjustment. You could accidentally break off the very hard, and therefore brittle, tip.

They are usually preset for optimum fire. I have never had to set the gap on an iridium plug.

NGK Says:

The manufacturers say NO.
This is because most people do not know how to properly gap a spark plug, and the center electrodes on the ultra-fine iridium can easily snap if mishandled. There is no warranty for snapped center electrodes. The manufacturers say an iridium spark plug will run so much better than a traditional plug, even if it is not gapped for that motor, that they would prefer you just leave it rather than risk snapping the center electrode.
Personally, we at sparkplugs.com, gap our iridium plugs for our own vehicles (we’re rebels). If you insist on gapping your iridium plug, please refer to Proper Gapping for instruction.
 
Last edited:
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SuperSix

Elite Member
Oct 9, 1999
9,872
2
0
I would put stock replacement plugs in it. There's arguably no gain from iridium on that motor.
 
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sontakke

Senior member
Aug 8, 2001
895
11
81
I suspect that when he puts back the old plugs, the car will still have the same exact problems.
 
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infoiltrator

Senior member
Feb 9, 2011
704
0
0
My feeling involves the plug wires if they are/have been in long use. Only changing plugs will tell.
 
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