Rough hesitating (hard) upon acceleration AE91

[teamyoda]

My car: 1991 corolla SR5

My car has been running perfect lately but just recently came up with a problem. I drove to the beach and when I drove home, the car hesitated and bucked really bad whenever I was accelerating. For example, if I was just starting out in first gear, it was a little rough but when I would shift into 2nd and 3rd, it bucked and missed really bad. Reminded me of our forklift running out of propane at work. This problem seems to diminish when I hit high speeds or high rpm's so I thought that would eliminate the fuel pump as a possible issue. So I did a few things already to try to identify the problem.

First, I checked for codes. There were none and the check engine light is NOT on at the moment nor was it at the time this problem started. I also jumped some terminals in the diagnostic plug to cycle the fuel pump while the engine is off. I heard a noticeable gushing sound and I could feel the pressure in the hose to the fuel rail when I squeezed it. So I think there is adequate fuel pressure. I also replaced the fuel filter. That did nothing. Took the air filter off, that did nothing. Also did spark test and replaced the plugs and wires/cap. Still no improvement. I even checked all the vacuum lines and they all were in good condition and didn't have any leaks.

I'm really out of ideas here... I thought about checking the timing, but would the timing cause it to randomly miss and hesitate only during acceleration? It idles just fine, perfect in fact. When I increase the throttle by hand slowly (under the hood) it does so very smoothly just like normal, but it seems when the car is under a load (starting off from stopped) is when the problem is at its worst.

I don't have a whole lot of money to just keep replacing things until I finally get it. School has made that impossible. So any help and guidance is greatly appreciated. Maybe a few things I could try just for the hell of it? By the way, the weather here has just been really hot, and is almost never this hot, only for the 3 weeks of summer we get. Thanks!

[BandoTRD]

recheck the vacumm lines for anything plug or blocking them. In some cases like for example mine when I bought my car it had a similar problem. It would start hesitating on low speeds but on high it would look to do fine. My solution was some carb cleaner to clean the intake after that everything was back to normal

[WhoaMan]

I know you said the problem appears to go away when at high speeds/high RPM... but does it appear to be affected by temperature at all?

Do you happen to have a vacuum gauge? If so, hook it up to the intake and check the vacuum at idle and when quickly opening the throttle plate. if it drops or stays near no vacuum pressure, check out your exhaust and catalytic converter for any plugs.

If the problem appears to change with the temp of the engine, inspect the EGR system, thermostat, and engine temp sensor

You also said that the check engine light has not been on, but have you checked for codes anyway?