RENIX off-idle hesitation

OK, so this has been a problem with my truck since I bought it. At throttle tip-in, the truck hesitates horribly, then takes off. I can avoid this by giving more initial throttle, but that's not feasible, practical, or safe for a number of driving situations(ie: traffic, parking, etc). I've done a bit of reading on here, but my problem has some conditions that others haven't seemed to have. First and foremost, this problem does not exist when the truck is still in it's cold start cycle. It's only after about 60 seconds or so of run time that it first rears it's ugly head. I'm not getting horrible fuel mileage(around 14mpg, which I feel is pretty decent for 30" tires and my heavy foot), and the truck doesn't seem to be down on power.

I had it in with my mechanic for some other things, and the only thing that he found that he says may be related is a low manifold vacuum reading. The specification range is apparently 15-22", and it's showing 15" exactly. I'm not sure if that is related or what, but I figured I'd mention it. Oh, and before anyone says it, the TPS has been replaced and is adjusted properly.

Any thoughts on this? I'd really like to get this worked out, as it's really taking away from my enjoyment of the truck, which I otherwise love. :helpme:

low vacuum can defiantly cause a hesitation. I would check your tps(throttle position sensor) as well as your iac(idle air control). here is a link to help out with the Renix sensors... http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

TPs will defintely affect this, but there are other things that can cause this. As far as IAC, off-idle will not be affected by this. Check your vacuum lines.

Check your TPS:

- Verify adjustment - it's been known to have them "walk" a bit when tightening to lock it in. Always verify after you've locked it down.

- Check function. You may have a defective unit. You'll need an analogue ohmmeter for this, you connect it to two terminals that should vary, and slowly run the sensor through its full range. Adjust the ohmmeter until it shows a nice big sweep, then check to make sure that it sweeps smoothly and evenly, co-ordinated with the movement of the sensor wiper. It's possible you have a flat spot in the carbon track that provides the variable resistance (I've seen two defective TPS units out of the box out of the several dozen I've replaced on various vehicles.

Also, check MAP output and function - you should be able to check it in the same way (it's also a variable resistance, but it changes according to changes in density of resistance medium.) Most of the MAPs I've seen let go have done so at idle or close to it.

As mentioned, check vacuum lines. Also, check torque on your manifold screws - particularly the ones around back that are used to fasten both aluminum and iron flanges. I could go into detail why, but suffice it to say that an "unscrewing" force tends to be generated there, and it loosens manifold screws over time (it's a good habit to check them about annually - you'll usually fine one or two having worked loose.)

A sticking EGR will cause this as well. It could be slightly open and stuck there.

Johnnie Walker said:

TPs will defintely affect this, but there are other things that can cause this. As far as IAC, off-idle will not be affected by this. Check your vacuum lines.

Click to expand...

I'm not sure about the IAC and off-idle. I believe that the IAC provides both the idle circuit and transition circuit that you are familiar with in a carb. Just my $0.02.

My vote in this particular case in the manifold bolts. Mine had a slow loss of vacuum and acted similar to yours until I re-torqued the manifold and it disappeared. My vacuum reading went to 20mm/hg and it has run great ever since. I now routinely check those bolts with each oil change.