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Discussion Starter #1 (Edited by Moderator)
EDIT/Cliff's Notes: This thread was originally called BAD Misfire, and the whole thread of diagnosis is intact, but if you go to posts 30, 31, and 39, you'll learn how to easily make your own smoke machine for finding vacuum leaks.

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Short story--

New distributor and all things related to it
New coil (old one used to rule out new one)
New Ford 9mm wires (only about 300 miles on them but you never know)
New Autolite 25s
All vacuum lines are new/checked
*13 timing advance
New IAC
New O2s
New map sensor
New intake gaskets
New TPS sensor and set correctly
Base idle reset
Codes pulled and repairs made.
Car is still SD (88 GT) The car misses randomly and badly during cruise and especially around 1,200 to 3k rpms. I can the tach jolt badly downwards when this happens. It feels like someone is bumping into the car it's so noticeable. I didn't replace all those parts at once but through diagnosis of running codes when I bought the car 4 months ago.

Plugs look pretty clean for 400 miles on them. No oil and no fouling. I checked spark at each wire. Odd though how only about 1 wire seemed to make a difference when I pulled it (idle drop) but it had spark. Exhaust doesn't smoke at all and no coolant loss etc.

If anyone needs more info just ask and I'll provide. Car is pretty ticked off right now.

Thanks in advance!



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what codes pulled and repairs made?

Do you have access to a scope, or datalogger?

Possibilities.

02
ICM
ECM
injectors
PIP/stator

and all wiring to it

Is this a totally stock '88?
 

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Discussion Starter #3
Codes pulled were O2, MAP, Tad (smog pump was removed by the previous owner) and TPS.

Distributor is brand new but I'm glad you mentioned these things because this did this with the old distributor and new distributor as well. It seems to have gotten worse since the first time I've noticed it in May. Injectors are stock original.

Car has headers, no cats and Cobra upper/lower from a 93', cai and all stock internals. Really not much done to this car.

I don't have a data logger. I also fixed the O/B hot wire from the starter relay to the ecm relay because the pump wouldn't prime. However, was still doing this before hand.

Thanks for the reply.

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and you have no codes now?

so all the parts you replaced pertaining to codes, has cured the codes?

It will be very difficult to diagnose without a scope or logger to see what is happening during the mis.

losing spark? losing PIP(crank signal)?, losing injector?

or the wiring.

do you have a vac gauge to look at?
 

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Discussion Starter #5
Sadly I tried my vacuum gauge and it broke while looking for vacuum leaks. Sorry, ICM was replaced about 3 months ago since the other was corroded. 82/22 have been fixed

Codes that are left that I'm having some trouble deciphering are: 28 (just got this one), 45 (may have assumed this is due to smog pump being deleted) and 88. All others were repaired.

I have a noticeable tapping around the 3/4 cylinder. Almost seems to be coming from the upper intake area. Almost like a loud injector?? The tapping wasn't there before but I really notice it now after raising the hood tonight and looking for arcing.



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Discussion Starter #7

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The car misses randomly and badly during cruise and especially around 1,200 to 3k rpms. I can the tach jolt badly downwards when this happens. It feels like someone is bumping into the car..........
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Primary Ignition: Does your tach jump wildly + or -approx. 1000rpms when you have the missing/stumbling issue? If yes, then you have an issue with your primary ignition components

Secondary Ignition: Does your tach jump wildly + or - 200rpms when you have the missing/stumbling issue? If yes, then you have an issue with your secondary ignition components.

From MSD:
The Primary Side:
The best way to define the primary components is that they are the parts that deal with the low voltage from the battery (the 12-volt side). Note that all of these parts use conventional wiring, since they’re carrying lower voltages (except the battery cables that connect to the starter and alternator). This includes the battery itself, the ignition switch, a switching device and the wiring that connects to the coil’s negative and positive terminals. The coil works both sides of the fence since it is the point where the primary and secondary systems meet as it takes a high voltage in but sends out a high voltage spark.

The Secondary Side:
The spark plug wire that connects the coil to the center terminal of the distributor (called the coil wire) is where the secondary side of the ignition begins. In stock systems, there is generally anywhere from 10,000 – 20,000 volts bolting through this wire. This spark zips through the wire and transfers to the rotor tip and then jumps across a small gap to another cap terminal. From there it moves out of the cap, through a spark plug wire, and finally across the spark plug gap. It is important to note that these secondary components, the cap, rotor and wires, are maintenance parts. Depending on your use, they should be inspected at least once every cruise season and numerous times during a race system.
 

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Discussion Starter #9
I'd say I'm more on the secondary side. A good jolt that knocks the tachometer needle downward violently by about 100-300 rpms.

I'm going to swap the spark plug wires and see if that makes a difference. I bought the Ford 9mm wires but hoping I don't have a bad set. I bought this car and drove it from IL without it never missing once. I've swapped back just about everything now but the wires since I added them after I did my intakes.

Thanks for the post!

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Discussion Starter #10
Put my other set of known good wires in and the problem still exists. I should add that if I put the pedal to the floor the engine doesn't miss. Always seems to be where I'm barely pushing the pedal, just enough to cruise. Doesn't do it when I accelerate from a stop or merge onto a freeway either.

Maybe I'm looking in the wrong area?

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Discussion Starter #11
I found a major vacuum flaw when I removed the upper intake. Research told me the MAP sensor vacuum hose needed its own dedicated vacuum hose from the intake. I had mine teed off with what I believe is the fuel purge sensor near the charcoal canister. From there, MAP's hose went into a tee that was fed from the intake.

I feel unsure about how I can properly route the hoses for this original 93' Cobra intake. I think part of my problem lies here.

I also found at the rear right most intake runner hole had gas on it and only that port of the gasket on both sides had raw fuel on it. Gasket seems very flimsy and maybe it failed. Intake was wet all around it.

All injectors Ohm wise checked out but I found one with a cracked cap on the intake side. Still doesn't rule out a mechanical failure of other sorts with the injectors.

Any help would be sooo appreciated....

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ohming the injectors just verifies they are not open.

tells nothing on how they perform under load.

if you feel you have an airleak, perform a smoke test...cheap and effective
 

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Discussion Starter #13
ohming the injectors just verifies they are not open.

tells nothing on how they perform under load.

if you feel you have an airleak, perform a smoke test...cheap and effective
I was waiting for a response from you.. Hahaha.... I agree about the Ohm test but as I stated, doesn't rule out a bad injector. As a former Ford tech, I rarely ran into injectors that wouldn't Ohm out right. Usually was due to mechanical internal failures.

I did even more digging yesterday about 93' Cobra hose routing and I found my repair I believe. The charcoal canister (which I do still have) has the hose coming off of it into the purge valve and a hose off of that. There's a vacuum port near the TB on the Cobra intake I capped off :livid: . So now I can put that there and that deletes the tee I used and gives the MAP sensor its own dedicated vacuum source off the intake.

I believe I had everything else correct. Going to check under the valve covers for any loose parts today.
 

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Discussion Starter #15

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Discussion Starter #17
its beginning to sound like a mechanical issue.
Mechanical as in engine internals or injectors? I figure as long as I have the upper intake off, I may as well check out the rockers etc.
 

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Discussion Starter #19
Updating my thread....

I replaced the lifters and pushrods. Found a collapsed lifter in the #4 cylinder. It also fixed the tapping but also an odd whirring/whistling noise too. Miss is still present.

I replaced the injectors with a set of #19 from a fellow member and the miss is still there. Compression tests were all close to each other as well. All wiggle tests have yielded no results.

I have two last thoughts before I tear my hair out:

1.failed spark plug from Autolite. Ive had this miss since I changed them and did my intakes. Noticed this issue since late May but has gotten worse. The #1 plug looks practically brand new compared to others. It looks white still around the electrode. I've found no cracks or carbon tracking.

2. Found my main ground from the battery to the timing cover in rotten shape. I don't see any breaks in the wire, so I doubt it would cause my miss.

This problem is definitely nailed down to act up (again, randomly) below 2k rpms, with barely any pedal pressure and in any gear. WOT is fine and very strong.

Any other insight Corral? I'm pretty much at a dead end.

Thanks.....


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