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'98 Prizm stumble on acceleration

'98 Prizm, 5spd, 100K. Pretty bad stumble whenever I try to accelerate, regardless of initial speed. Idles fine, Runs fine at speed, throws no codes. Replaced spark plugs, PCV valve, no change. Connected to OBD computer and data, like MAP, spark advance -- changes as you would expect under acceleration. I have put some injector cleaner in the gas now but that is a stab in the dark. This is my daughters car so I only have a few days to figure this out so I would appreciate some suggestions to pursue. I can't find an EGR valve on this vehicle, otherwise I would look there.

Seems to me that when the vacuum drops, the problems begin, then recovers as load is reduced.

Thank you in advance....

Travis

Reply to
Travis
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If you did not already, try cleaning the mass airflow sensor (hotwire). Use a spray contact/brake cleaner that does not leave any residue. Do not touch the sensor with your hands or use swabs, just spray it clean. That may help.

Reply to
user

No MAF can be found. I looked through the factory manual set and there is no mention of it. There is a MAP sensor on top of the throttle body, what I think is the IAC idle air control, below that. In the air filter box the only sensor is for air temperature. A small probe above the filter element.

Reply to
Travis

Jeff, I think that makes a lot of sense except on this model the pressure regulator is part of the pump assembly in the tank and there is only one line out to the fuel rail. There is no connection to engine vacuum as near as I can tell. The pressure is not monitored by OBD and I do not have a mechanical gauge to verify that the PSI is being maintained.

Thanks, Travis

Reply to
Travis

Start with the basics.

Check the condition of the air filter.

Check to make sure there are no cracks, gaps, holes, etc. in the black plastic snorkel between the air filter housing and the throttle body.

Check manifold vacuum to make sure there are not vacuum leaks.

Check the condition of the ignition wires.

Reply to
Ray O

OK, Basics -- spark plugs -- I had the old NGK originals that I replaced several years ago with Autolite "equivalents". The stumble seems much reduced after regapping the old NGK's and putting them back in. I think I will go to the local parts store tomorrow to find Denso or NGK's.

Thanks, Travis

Reply to
Travis

Let us know how you make out!

Reply to
Ray O

Take the cheap way out. On an '85 Toyota truck and a '90 240SX that had this problem I found the fuel filters were full of mud. Low speed acceleration was OK on the 240 SX, but low speed and high speed acceleration on the truck was horrible, and high speed acceleration on the SX was pretty bad.

Reply to
Hachiroku $B%O%A%m%/(B

Well, I was disapointed this morning that the stumble was back and almost as bad as ever. After thinking about this for awhile I worked out the problem. It seemed to be related to the spark plugs but both sets perform about the same. One of the plugs (#4) that I first removed had a curious black track down the side of it. This plug was also a bit carbon fouled where the other

3 were lean-white. I looked hard at the plug wire connector that goes over the #4 plug and there was a carbon arc line straight down the inside that hugs the plug insulator. I scraped it with a pocket knife and the stumble is gone. I will order a new wire set for good measure.
Reply to
Travis

Thanks for the update! Stick to Denso ignition wires ;-)

Reply to
Ray O

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