Low RPM Bucking/Sputtering Under Load

[Brook24v]

1994 MTX, 289,000 miles

I'm at the end of my rope here and looking for any kind of suggestions. A while back the car would sporadically buck and sputter under load until about 3,000 RPM or so. I'm talking a hard buck/misfire with intake pop/backfire sounds. If you let the clutch out and barley accelerate like a grandma until 3,000 it would run 100% normal and also run fine past that. Well the sporadic turned into every time, while above 3,000ish RPMs still pulling hard and smooth. Now not only is it doing this every time on the low end, but even above 3,000 RPMs starting to sporadically not pull hard, although no bucking is present. It is the weirdest deal. Any thoughts? I have done the following:

• New plugs 2,000 miles ago
• New wires 2,000 miles ago
• Just today swapped old wires on and duplicated the issue.
• Pulled new plugs out and inspected, set gap at .045. Duplicated the issue.
• Swapped MAF out with known good one. Duplicated the issue.
• Swapped Coil pack out with known good one. Duplicated the issue.
• Unplugged cam position sensor, duplicated the issue.

I really can't think of anything else. Would a bad engine ground cause this issue? I actually don't know where the engine ground is on these cars. I considered the crank position sensor, but I have doubts on that one. Help!

 

[Irish Pride]

Crank Cancer? Check your Timing Sprocket for play.

 

[SHOdded]

To quote a broken record, "did you pull codes"? The most popular ground to check connects the rear left of the intake (looking at the engine from the front) to the firewall. Seems like the DIS is the only non-swapped part in your list

Jimtash had an intake pop issue recently, so his thread on this might be illuminating.

Could be you need new cats/O2 sensors too.

 

[zblackbeast]

Could be the coolant temp sensor.

I would start by pulling codes and check fuel pressure Brook.. what did the plugs look like when you pulled them?

 

[Brook24v]

- Plugs looked pretty good with just a hint of white on them.

- I forgot to mention that I did swap a DIS module and rechecked firing order.

- Would a crank sensor allow this issue to happen, or just simply not run once the sensor is INOP?

- Actually that ground cable was unhooked from the intake. I couldn't remember where it went so I attached it to the upper intake bracket bolt. Is that right?

- I can pull codes. I have not done this yet since a light has not flashed for some time.

- 02 sensor would not surprise me since it has thrown a code in the past.

- IMO a bad cat would cause more issues in the upper RPMs apposed to lower.

- I never thought about a coolant temp sensor. I should check fuel pressure. What pressure do these run at?

 

[zblackbeast]

I think they are around 55psi or in that range, but that's just a guess.

The ground strap goes from the rear passanger support bracket to the firewall. This ground the pcm, with out this the car will not start. I, and most people, move it from that support bracket to the rear crossover mounting bolt.

from my experience and what I've read a crank sensor -usually- is either good or bad. Either it will spark or it wont. With my random bucking. It was the cam sensor, but you've already replaced this, and its only at low rpm which leads me to believe its a fuel issue and not a spark issue.

Replace the coolant temp sensor (about 12 bux at advance) and go from there. My 92 parts car had the same issues and after replaceing all the ignition components it turned out to be that sensor.

As far as codes, just because the light isn't on, doesn't mean there isn't one stored. I would run the KOEO and KOER codes and make sure they are both 111!

Good luck, if I was still in Omaha I would drive down to help, but I can't ssadly.

 

[Brook24v]

Thanks for the advise ^

Random update: I've been driving other stuff and kinda put this on the back burner until today. I pulled the codes and got the following

157 MAF low voltage
172 HEGO H02S sensor fault/lean
176 HEGO H02S sensor fault always lean. Insufficient TP change during no o2 sensor switch
214 Cyl identification failure
542 Fuel pump secondary failure

I'm assuming the 172 and 176 is o2 sensor codes? The 157 and 214 confuses me because I checked the MAF and cam sensor (if that applies). The 542 surprised me. Not sure what that one means. Thoughts?

 

[intimdatr]

Hows your TPS? Original? It might be getting tired.

 

[Brook24v]

Man, I can't think of the last time I touched the TPS. Good question. Haha

 

[NEp8ntballer]

does the tach work right? Cyl ID sounds like the crank sensor to me. sounds like fuel or spark though. how old is the fuel filter?

 

[Brook24v]

The tach does operate correctly. How often does an SHO crank sensor act up though while still actually allowing the vehicle to run? I would think it just flat out wouldn't run. I don't know.

Everything spark related has been replaced, so next up is fuel. I'd have to look through records to see how old the fuel filter is. Either way I'll throw one on for the **** of it.

 

[rubydist]

your symptoms sound like the tps is a good candidate for being ill. the tps has (imho) an overly important role in how much fuel the engine gets, so a sick tps can result in lean misfire to the point of basically having no power, just like you describe. in many cases, a sick tps with symptoms similar to yours will not result in a code being set.

the crank sensor can fail intermittently, however, every single time I have seen that happen, there was a stored code for the crank sensor. the first symptom in some cases is occasional sputtering / missing when the engine is warm. this is followed quickly by engine quitting when the engine is warm, in my experience. your symptoms do not match that.

 

[vortex2450]

I am going to agree with the overall consensus of the issue being a tired TPS. As with my other SHO owners this has happened to someone else before at least a dozen times; I had some similar issues a while back and swapping the TPS was the ticket.

One way to get a better idea of it is to try various throttle positions around 3000rpm and observe how the engine reacts to the different throttle positions in that performance range.

-josh

 

[VortecGT]

I have the same problem right now, this had happened to be in the past and it turned out to be two bad plug wires. Even if they are new use an ohm meter and test each wire.they should all be almost the same but the longer the wire the more resistance.

 

[Brook24v]

Tried a new TPS. Didn't really change anything. I pulled codes again and had a 114 air charge temp sensor fault code so I grabbed a new one. That actually seemed to fix at least 50% of my bucking, but it's still there. The car can at least pull to redline every once and a while now without any issues. After beating on it a while my RPMS started surging pretty bad. I wonder if my IAC is ******.

So, what's next? :laugh_ti:

 

[rubydist]

the iac will not make the rpms "surge pretty bad" except at idle.

reset all the codes and drive it a couple days, then read them again. the 114 is pretty bad in that I have never seen one of these engines run right with that code present.

I'm starting to wonder if you have an intermittent wiring issue. I suggest cleaning all the grounds you can find.

 

[Brook24v]

Well it still runs like crap. That air temp sensor didn't improve anything. It was just in my head I think. I threw a fuel filter on for the **** of it. Didn't do anything. I'm slowly catching this thing up on long overdue maintenance by throwing parts at it, so all this stuff isn't bothering me too bad. Saturday I'll check fuel pressure and throw on a couple 02s since I know the ones on the car are flashing an engine light.

The surge is only after driving it for a while and whenever I push in the clutch where it supposed to be idling. The engine does not surge when it's cold.

 

[rubydist]

some iac valves will work fine cold and stick hot - others will be the reverse, and some will stick all the time. your latest description does sound like an iac problem.

I still think you should get a tps too.

 

[sho'noff]

what octane fuel do you use? I have noticed octane boosters typically are sold that only raise the octane one number- 10 points = one number. too high octane will do that, it just wont ignite.

 

[Brook24v]

I tried a TPS last week. Didn't improve anything.

This car has only had 91 ran in it since the late 90s. It's been bucking for quite a few tanks now, so I'm not to worried about the quality of the fuel at this point in time.