I have a '98 Jimmy. Just replaced fuel pump, fuel lines, fuel filter, spider & spark plugs. Cleaned everything in sight incl MAF.

Recently driving on highway, the slow driver in front of me found their turnoff and I accelerated to get up to speed. I noticed a very slight hesitation, almost too slight to notice. Next time I was on the highway, I paid extra attention and there was an actual hesitation, slight but noticeable.

There is no CEL. Found close but not the same with search. Any ideas?

 

There is a long list of possible causes, everything from fuel delivery to TPS sensor issues to spark strength or delivery. Even compression. We need to narrow down the possibilities or it will be the parts cannon.

What problems were you having that prompted the work that you described? Did you have any codes at that time?

Have you confirmed proper fuel pressure and leak down after the repair?

Are there any pending codes?


George

 

I had mis-fire code p301, prompting plug change. Coolant leak search convinced me that LIM gasket would be a good idea in any case. While doing this, seemed only logical to change to MFI spider. Leak turned out to be in a rad hose, but LIM gasket was damaged, so no harm, no foul.
Fuel leak was going to require dropping the tank, so new Delphi pump went in too. Fuel leak turned out to be return line, so all new fuel lines went in.
CEL was on cos of fuel leak, but shut off with scanner after replacement. Haven't yet checked for new codes or fuel pressure.

 

A good diagnostic flow would be start simple and get more involved. Occams Razor and all that. In order:

1) Any codes or pending codes? Any freeze frame data?

2) Any difference in performance cold vs warm or open vs closed loop?

3) 1" of strong blue spark at the plugs?

4) Inspect for vacuum leaks and ignition problems (cap, arching, wires, connectors)

5) Look at the basic sensors cold then warm: TPS, IAT, ECT, MAP, MAP.

6) Look at fuel trims and O2 sensor graphs in closed loop at idle, 2500 and WOT

7) Verify fuel pressure and leak down.

I'm the diagnostics guy not the parts cannon guy. If you want any help with these let me know, I'll be glad to help.

George

 

I'm planning to scan and pressure test fuel today. I'll be back.

 

In the past I had an issue with my tps sensor seemed I had just knocked it loose after plowing through a snow bank but as it was acting up my blazer would lag and then burst with power and would also not rev properly in neutral or park also liked to stall out I hope this helps

 

I'm back with a followup. The scanner showed nothing out of the ordinary and fuel pressure is okay. Haven't done any spark plug checks cos of lack of indoor space and nobody much likes working on frozen vehicles. The plugs were new a few months ago.

For the heck of it, I dumped a can of cat cleaner in the tank and filled it with 90 octane. I'm only about 1/4 down the tank so it may take a while before any benefits show up. I had thought the “stumble” went away, but it showed up briefly yesterday. Again while at highway speed and accelerating, like the cruise kicks in going up a grade.

It's Watch and See, I guess.

 

1) Did you make sure that your TPS has good smooth output through the full range with engine off?

2) You will need to look at your O2 sensors and make sure that they are not sluggish and responding poorly

3) it can also be poor response from other sensors like the MAF under load condition which can be monitored with live data

3) You will need to look at fuel trims under various conditions. This will help identify spotty fuel injection delivery, slow sensor response, dynamic fuel pressure, etc. Related tests can be fuel pressure while driving, injector balance test.

4) As mentioned before you may have intermittent or weak spark

5) You can also have exhaust restrictions such as the cat. There are a number of tests for this but the easiest is a pressure gauge in an O2 sensor socket

I realize that some of these tests are more advanced but I'll lay out a plan for you to consider.


George

 

BTW, a cat cannot be fixed, particularly if that's the cause of your stumble because that would be a cat crumbling and mechanical pieces blocking flow causing increased back pressure. As a matter of fact what kills cats are unresolved fuel delivery issues.

George