Random bucking on boost

Jayls5

Registered User
I just bought a 5-speed supercoupe, and I have discovered today that it bucks when getting into boost.

It doesn't happen every time. On occasion, it just seems to stop entirely during hard acceleration.

The guy supposedly installed a new cam-sensor. I'm wondering if you guys have any suggestions as to what it could be. Thanks.


Edit: plugs were also supposedly recently replaced
 
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I would double check the plug wires make sure they are properly seated. The plug could be the problem with hard accel bucking.
 
my black 90 had a problem just as you described when i bought it the problem was a faulty ignition coil pack
 
my black 90 had a problem just as you described when i bought it the problem was a faulty ignition coil pack

I have a sneaking suspicion that is it, considering he left a junk-yard coil pack in my trunk. I'll be asking him tomorrow whether it's one he already changed out or if he had planned on swapping it.
 
If I had to bet on it I'd bet your problem is plug wires or plugs. Even though you say the previous owner supposedly changed the plug wires some brands of plug wires just don't work well on an SC under boost. Also I hope your running at least 92 octane gas. Lower octane gas (87, 89) is a big no no for an SC under boost.
 
If I had to bet on it I'd bet your problem is plug wires or plugs. Even though you say the previous owner supposedly changed the plug wires some brands of plug wires just don't work well on an SC under boost. Also I hope your running at least 92 octane gas. Lower octane gas (87, 89) is a big no no for an SC under boost.

I just refilled it with 93 octane today. It still does it sporadically.

Since the storm's gone, the roads were dry enough to pay attention to the gauges today. The tachometer drops to zero when it bucks. (It essentially just stops dead in its tracks and then continues driving normally after the first bucking motion)
 
Update:

I haven't replaced the coil pack yet. However, I have had 2 occasions in which the car sputtered and stalled upon starting.

Otherwise, idle is perfectly level and around 23 ish on the vacuum gauge. I'm inclined to think this isn't a vacuum issue since it runs normally on most occasions. What's the normal vacuum level?

I also discovered it boosts 14 psi, despite being told it's stock (and the pulley seems normal sized). Is the exhaust possibly clogged, causing the extra boost? It pulls nicely when it's running right.
 
Update:

I haven't replaced the coil pack yet. However, I have had 2 occasions in which the car sputtered and stalled upon starting.

Otherwise, idle is perfectly level and around 23 ish on the vacuum gauge. I'm inclined to think this isn't a vacuum issue since it runs normally on most occasions. What's the normal vacuum level?

I also discovered it boosts 14 psi, despite being told it's stock (and the pulley seems normal sized). Is the exhaust possibly clogged, causing the extra boost? It pulls nicely when it's running right.

Vacuum is normally around 21 inches on a stock motor but can be off a couple inches either way because of the gauge not being accurate...same goes for the boost.

I'll bet you have a loose or bad plug wire or a problem with one of the spark plugs, like a cracked insulator. When weather is wet, the problem will usually get worse if that is the case.

David
 
I would pull each plug out and check them. i had the same problem on my 92 and one of the plugs had a crack in it
 
The guy supposedly installed a new cam-sensor.

Edit: plugs were also supposedly recently replaced

"Supposedly" might be the operative word here. You mentioned on another post the tach drops to zero. Had similar symptoms even after new plugs/wires. Car is bone stock '90 AOD with 98,000 mi. Searched here for "tachometer" and found several posts connecting cam sensor to this issue. Had mechanic install NEW cam sensor, even though he didn't find a problem when diagnosing, problem disappeared, has been couple months now.
Another indicator is the intermittent occurrence. Bad plug(s)/wires will probably do it on boost very consistently, but I think the tach sensor, being a magnetically coupled device, is probably temperature-sensitive and won't start to act up until it's hot enough.
Even though the sensor is supposedly only read during startup, my hypothesis is: DIS loses signal, or maybe "resets" on loss of signal, reverts to "guessing" on TDC for firing, and winds up sparking on (unburnt) exhaust stroke, causing SEVERE bucking.
I also bought a NEW FORD DIS just in case it was needed but it hasn't been so far.
 
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Even if the plugs were replaced recently...that still makes the wires suspect. It's not unusual to damage a wire, just by handling them when changing the plugs. Had the same thing happen on my car after installing my first set of headers. It ran great for a week then suddenly it was like a bucking bronco whenever you got into the boost.

David
 
I just refilled it with 93 octane today. It still does it sporadically.

Since the storm's gone, the roads were dry enough to pay attention to the gauges today. The tachometer drops to zero when it bucks. (It essentially just stops dead in its tracks and then continues driving normally after the first bucking motion)

I can't imagine that the tach would cut out for plugs or plug wires.

The #1 suspect, in my opinion, is the DIS ignition module. Not the coil pack, but the little gray module on the passenger side of the engine bay.

Just in case you have any codes present, be aware that "Coil pack circuit #x" codes can also be caused by the ignition module. Very sneaky, I know. I replaced a coil pack for no reason once because of this.
 
Ok, so I replaced the coil pack. The problem is still there.

And I should reiterate, this is random. I'll be boosting fine for a while with no issues, then it will randomly happen. I've also discovered that this doesn't necessarily happen under boost or major load. I've had it happen just easing into first still in vacuum.

I took off the MAF, and I didn't see any junk accumulated on the coil.

I'm thinking the next thing to replace is the DIS.
 
I'm tellin ya dude, Cam Position Sensor. Even my mechanic couldn't see the connection. It was going on for almost 9 months, first would only do it if I got into the boost just a little. Later on would do it just cruising at 55 in vacuum, and tach started dipping for a second when it happened. Tach would hiccup when it was happening, just a momentary dip. Finally one day it started bucking REALLY badly and tach just dropped to zero for about 45 seconds. Motor was still running. I was pulling off freeway in case it died when tach came back and car acted normal again all the way home. 2 days later got Cam sensor replaced and it's been fine ever since, over 1000 miles now.
Let me also point out that the bucking, when it's bad, definitely ain't the same as a cracked plug/bad wire, cause I had those problems before too. With a bad plug it would go up against 5lbs boost and then you could watch the boost gauge needle tapping on 5 lbs while the plug kept shorting to the block in perfect rhythm. You could drive it in vacuum and never know a plug was cracked.
 
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For my first post I will just say that it is great to have an online community to discuss these problems with. I love my 1990 black SC which I picked up 5 months ago. It is AOD and I am dealing with a similar problem as well. I have spent hours trying to figure this problem out. I have narrowed it down now based on heat. BTW, I replaced the Cam Sensor today and it did nothing.

Cold start is perfect and runs perfect, power is all there even for long hard drives. If I turn the car off and leave it sitting for about 1/2 an hour, it will start rough and sputter and almost die. If I rev it and then let it smooth out and then drive, it will go for a bit and then buck like crazy with tach dropping a bit etc. What I have to do at this point is to turn the ignition off and start the car again, if it idles rough I have to do it again and again untill I get a smooth idle at start. After a smooth start, it will run fine untl Iturn it off and let it sit for around 1/2 hour again.

I am now going to look into DIS. Seems like a possible choice. BTW, if I leave the hood open for that half hour and let things cool down easier, there is no rough idle or any problem at all.

I can usually avoid this by paying attention when I start. If it idles rough when I start, I turn it off and try again untll I get a smooth idle at start.
 
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