1989 Plymouth Horizon • 4 cylinder FWD Automatic • 35,000 miles

Plymouth Horizon
1989 Plymouth Horizon Hesitation when first increasing gas pedal. Happens at all speeds all rpms, Hesitation and a little bit of vibration, once I give it more gas it is fine. 35k miles, just put on new water pump, timing belt, cap, rotor, plugs, wires, fuel filter, air filter, belts, checked pcv valve, gas is new. Does it on multiple tanks. Tranny is fine had it adjusted months ago. Has power. Also idels little rough. Had 2 garages adjust timing. I do notice carbon buildup on the part of wires that is in the distributor, so I cleaned them up and put white grease on them. Ran a little better but I think once the carbon built up again it goes back to same problem. Ran fuel injector cleaner multiple times, nothing. Plenty of power on high revs so not a clogged cat. Any suggestions? Thanks! Thing is in Mint Condition.
July 19, 2011.

Use Brakleen or carb cleaner to check for a vacuum leak. Spray the cleaner product on all vacuum lines and fittings. Also spray around the intake manifold gasket. Don't forget to spray the fitting going into the brake booster. If a leak is present, the engine will surge when you spray the cleaner. Also check the throttle position sensor, using a multimeter, to make sure the resistance readings change smoothly as the throttle is opened and closed slowly.

Also, remember that injected cars have a small throttle hesitation built in, due to lack of an accelerator pump. In my carbureted opinion.

Aug 13, 2011.
Sorry put flag for review on the answer.I thought that was the way to add more info to the answer for additional feedback.

I should correct my definition of the problem . When cruzing at a lower RPM s 1000-2200 when I first give it a little gas it bogs down and will continue to bog down until I give the car a little more gas. Once I give the car more gas it is fine. It also idles a little rough and seems to ping to my ear but mechanic says that is a normal pinging sound. I have decent power and get 31mpg. It will rev up to 4 grand fairly easy. It is just a pain giving it more gas each time it only needs a little as I end up speeding up then down, up then down.

Also could this be a related problem and if not what is a possible solution? I had to disconnect the air control valve on the air filter cover because it would close and stay closed (it is usually open when not running) thus only bringing in heated air from the exhaust manifold. The tube that went from the air control valve was getting vacuum, it went to a small round disk closer to the TBI on the air filter cover. That disk has two vacuum inlets, one for the air control valve and one that runs to the canister purge valve solenoid. What can fix this air control valve so it opens when the car warms up?

So Far I have replaced and checked the following
Fuel Filter
Coil, Plugs, Cap Rotor. Plugs are champion rec. By dealer.
Timing done by garage.
Air Filter
Knock Sensor (there is none)
Air Change Temp Sensor
Fuel Injector
Map Sensor
Idle Air Control Valve
Gasket under Throttle Body
Throttle Position Sensor
Timing Belt
Computer was replaced a few years ago
Checked EGR Valve
Checked for vacuumed leaks several times
Cleaned Throttle Body
Passed emissions
Fuel is super with some Lucas Fuel additive

? Could it be the
1. Distributor Pick up Plate?
2. Could it be the EGR valve, I had someone else check it? Could it be the timing, I did change a lot of sensors since it was done but the problem still existed before and after?
3. That air control value issue, though it ran the same when it was open or closed.
Anything else ?

Thanks for the help

Sep 25, 2011.
If I'm thinking of the right air control valve on the air cleaner housing, it's thermostatically controlled, drawing in warmer air off the exhaust manifold to help the engine warm up, and once warmed up properly, it diverts to draw air from the outside. Make sure the moving parts inside this valve operate properly with no binding or sticking. Check the throttle shaft on the throttle body for play.

Yes, it could be timing related, not advanced far enough. Yes, there is a possibility the EGR is having a minor effect.

Check the throttle position sensor with a voltmeter. At idle it should be dropping.16V and at full throttle it should be dropping 4.7V. If not, replace it. With the engine off, check the resistance of the TPS throughout its range of motion.

Check the timing, at idle, off idle and full throttle.

Sep 26, 2011.
Looks like it was the EGR Valve. Car runs much better now that I replaced it. Thanks

Jan 15, 2012.
Our pleasure. Tell your friends about us!

Jan 16, 2012.