Just got the '87 out for the first run of the spring and experience some "bucking"/"missing" while driving. This only happens at engine speeds around 2K rpm and higher and only at steady cruising (light throttle). Open the throttle a bit and it smoothes out. It starts and idles perfectly. Pulls strong with no missing. Normal in all other driving regimes.

Over the winter I replaced plugs, wires, cap, and rotor. All Delco stuff, plugs gapped correctly, etc. Engine is all stock.

The symptoms lead me to believe vacuum leak. I suppose I may have disturbed a vacuum connection while doing the ignition work. I also replaced the AIR pump at the same time.

I've taken the ignition components apart and put it all back together, making sure everything is tight. I can't see any obvious vacuum lines amiss or kinked. Any thoughts on where to look?? What's the best way to search for vacuum leaks? The ether trick? Stethoscope?

Thanks for any help!

 

There's a vacuum line right there near the distributor, and a few that run under the plenum. Quick visual will tell you if one is broken. Otherwise, try spraying brake cleaner around the vacuum lines while the engine is running and pay attention to any changes in idle. Have a fire extinguisher on hand...just to be safe.

 

Thanks, scooter. I know the vacuum line you're talking about. One of those connections did get undone when I was working in there but it's hooked up now. Nothing seems out of place but I will check again. I do have a can of carb cleaner that I'm assuming would work equally well for checking for leaks?

 

No luck with the carb cleaner spraying at the vacuum joints in the vicinity of the distributor, etc. I did it during open loop (cold start). No CEL or anything of that sort. Any other ideas? The fact that it idles good, has good power, pulls strong, and is smooth at lower rpm leads me to think it's not ignition related but is that a false assumption? I even did the "dark garage looking for sparks while running" trick and didn't see a thing.

 

throttle position sensor comes to mind, as it would be used (up) in the RPM range you cited....but given the work you did and the sudden onset, i'd look vacuum again.

 

Since you were in the distributer I would check if something is jamming the vacuum advance

 

These don't have vacuum advance do they??

 

Does this only happen when cruising? Cruise control engaged or not engaged? And only at the indicated rpm? Applying throttle relieves the problem; does letting off the throttle do the same? What is your speed when this happens? I'm being to think the problem lies elsewhere. When you say the car is "bucking" , is the car speeding up and slowing down as though it can't decide what rpm to settle into? Or does it feel like it's losing power, then surging, then losing power again as though it's starving for fuel or air? Not ruling out a vacuum problem, but I'm also thinking a vacuum leak would also affect engine idle. Check your O2 sensor. How old is the fuel filter? Check for cracks in the air duct between MAF sensor and throttle body. Air filter...how old? Also, What is your voltage while this is happening? Should not be too low or high. Do you have access to a scanner? We'll figure this out. Hang in there.

 

I have a 94 so don't know but would check where you were

 

It happens with or without the cruise control being engaged. Anything from about 2K rpm or above but with relatively low load. High rpm is fine when accelerating harder. I can also make it happen by just revving the engine in park. Get to about 2K and it happens. Yes, either accelerating harder or letting off the throttle makes it go away. I guess "bucking" is not a good word for it. It's more like it's missing/cutting-out. When I do it in park, I can actually hear some spitting/backfiring too. Not extreme but it's there. When driving with light throttle and I get to about 40-50 mph, it happens because that's the right rpm range in 3rd (automatic). When it shifts into 4th and the rpm drops, the problem goes away....until I get to about 65-70mph when the rpm is back up over 2K.

Fuel filter has some age to it (10+ years). The air filter is virtually new. Haven't checked to see what the voltage is doing when it's happening. I noticed it in the 13.6 range otherwise. It was running fine last fall so I've been focusing on just the things I've touched since then, which is plugs, wires, cap, rotor, and AIR pump. I did remove the air filter housing, MAF, and ducting so I'll double-check that for any cracks, etc.

Thanks for the help! It's a weird deal for sure.

 

A little late to be asking, but, have you checked for any fault codes?

 

I haven't checked for codes but the CEL has never come on. I just checked the intake ducting, triple-checked the plug wire routing, etc. Nothing. I did just start the car cold and the problem does not show up in open loop. After it warms to the point of going into closed loop, it happens. Starts missing at just under 2K but if I keep opening the throttle, it will smooth out at about 2600-2700 rpm. This was just sitting in park.

 

Oh and I didn't see any voltage variation when the missing is happening. Stayed at 14.1 the whole time.

 

So, problem only occurs in closed loop. 2000 rpm in open loop, cold engine, and there is no problem. Check that O2 sensor. Sometimes folk will put sealant on it or loctite when they install or reinstall after removing. This can burn off the threads and find it's way onto the sensor. Will look like a white or silvery powder coating on the sensor. This will affect the signal it sends to the ecm. The ecm uses this signal to control air/fuel mixture. It the signal is erratic, your air/fuel mixture will be also. Do you have access to a scanner? You can read the O2 sensor voltage with it. Also, do a fuel pressure check. Should be around 42 psi with engine off, ignition in run. Start engine. PSI should drop anywhere from 3-10 psi. Should drop further as rpm increases. If it doesn't you may have a problem at the pressure regulator, which would be a vacuum problem. First, check for codes. Then pull the O2 sensor and see how it looks. Proceed from there.

 

Ok, I'll check for any codes. I can get a scanner but for now I'll have to put the jumper in the ALDL. It's been years and I can't find my notes on which pins to jump to read the DTCs. Do you know offhand?

I replaced the O2 sensor about 8 years ago or so and haven't touched it since. Still wouldn't hurt to check it I suppose.

Thanks again!

 

A and B. Top 2 right hand side.

 

No codes.

 

...... No vacuum advance on EFI motors .......

 

...... Check the timing ... set the TPS ... and check the EGR valve .....

 

Check to make sure that the spark plug wires are snapped onto the spark plugs and distributor cap terminals. In my experience, high secondary resistance can cause this. Maybe the term "fish-biting" is a better way to describe what it's doing?