I'm having a few problems with my Blazer. Here's a little history.

Last summer(2008), I replaced the fuel pump with an Airtex pump. Since the install, I was having an issue with the truck hesitating. Like I pulled my foot off the gas for a split second. RPM would drop slightly. Mostly noticeable while cruising at a constant speed. I learned to live with it, and also figured out that if I pulled over and shut the truck off, it usually didn't do it after I restarted it. That pump crapped out on me, so I replaced it with a Carter pump and new inline filter 3-4 months ago. Right after that, I replaced poppets with the updated injector and fpr. Also replaced distributor, cap, rotor, plugs, wires, and coil at the same time. The hesitation problem has been gone ever since I installed the new pump. Two or three weeks ago, I replaced the Duralast battery with a Delco.

Fast forward to today. I took the Blazer through an automatic car wash. When I left, the hesitation had returned I also noticed once when leaving from a stop sign while watching the tach, when it got up to around 5k RPM it jumped rapidly to about 6k RPM, but engine RPM did not increase. I'm not sure if the two are related. It never did that when it had hesitation problems with the old pump. Any ideas on what may be going on here are appreciated. Thanks.

 

Do you have a SES light on?

 

No SES light. When it does come on, it's p0420...which is tied directly to me deleting the cat a few months ago.

 

I would lean tward a crank or cam sensor but could be something as dumb as a lose ground.

 

Sounds like you could have an intermittent issue with the CKP (crankshaft position) sensor and/or it's wiring. The car wash affecting things could be a result of water getting into a connector or into a harness where it temporarily shorts a connection. The tach is run off of the CKP sensor input to the PCM and this could definitely cause a hesitation.

Depending on the duration of the occurrence (the intermittent issue), this problem may or may not cause the CEL to be triggered. But if you think your CEL is only on due to the P0420 code and haven't had it checked more recently, you really do not know if there is any other codes stored.

 

I have a code scanner in the truck at all times. That's the main reason I bought it. I got tired of going to Autozone every time it came on. No codes other than p0420.

I'll probably wait and check it out when I get back to class in Jan. We're covering driveability, which this seems to fit into. I'll probably rewire the fuel pump at that time, also. That was one thing I forgot to mention. Before I took it through the auto wash, I sprayed off the wheel wells and rear end. My spare is in the truck instead of underneath it ATM, so it may be possible that I got water into the connections at the pump. They need to be soldered and wrapped with heatshrink. I didn't have a soldering gun at the time, so I crimped and wrapped with heatshrink that wasn't the proper size.

Edit: it seems that the hesitation issue may be gone. I put 40-50 miles on it today without a single problem. I still intend on rewiring the pump, though. I'm pretty sure my timing cover had a small leak that I need to address, but I'm not ready to pull the engine just yet. According to AllData, that's what I've got to do to replace the cover...or at least to pull the oil pan which you've got to remove to replace the timing cover.

 

So I think the tach jump problem is all but gone. I pulled the CKP, cleaned off the junk on the end, and reinstalled it. Did a CASE relearn with the Verus and I haven't seen it happen since. I did that about 2 weeks ago. CEL was on and it was either P0335 or P0336.

The hesitation is still there. Pulling out into traffic and it felt like I slammed on the brakes for a split second. RPM's dropped, then went right back to ~4200. Still trying to figure this one out.

 

Have you tested your TPS sensor to make sure that it is not dropping out somewhere between idle and WOT?

 

Not yet. That was my next step. It's an intermittent problem, though. I've taken it out and datalogged with the Verus on numerous occasions on never can find a problem when it actually does it.