strange hesitation - SOLVED!
 

feels like around 50-55mph my burb has an occasional hesitation like its not getting enough gas. I hit the gas and its still hesitating for a few seconds then kicks in. It seems to only do this while driving 50-55mph and cruising (not pushing gas but not let off, just enough to keep current speed).
It is my 91 burb in signature below, 454 Tonawanda R2500 TBI.
what might cause this? ideas?

I've already replaced feul filter, spark plugs (Bosch Platinum), sp wires and not much else. Recently had a oil change (reputable place) with Penzoil 10W30 and except possible need for rear u-joint and rear brakes everything checked out.

:fb:

I would say timing, check dist for slack- good luck.

so this is it? i was wondering if a fuel pump about to go might do this or what else it may be

Here's what I'd check:

*EGR - could be carboned up and sticking.
*Injectors - could be dirty
*Vacuum leak
*Dist. Cap - could be cracked.
Timing - Local garage timed mine without disconnecting the diag. lead...ended up being really sluggish until I showed them how to time it properly.
*Throttle Position Sensor - this may or may not set a code.
*Coil - maybe, though unlikely.

I hope this helps!

thank you i appreciate the assistance 87SC

edit: i will tackle what i am able to but as i am not a mechanic, i will stick to dist cap/rotor, vacuum lines but for the timing will have to be taken to the shop...

I'm betting on the EGR given the symptoms you describe. Sounds like it is sticking and not closing right away. If you need to replace it, go with a GM unit, the NAPA ones don't last.

Good Luck!

i appreciate it, i guess after 160K miles it is due for some maintenance

ok i did some digging around and here are some other responses from other people having same problem as myself:
starts with -
"We just bought a 1991 2500 3/4 ton Suburban. Yesterday it started missing (I guess that is what they call it) As you drive down the road between 45 55 it starts bucking almost like it loses power. when I speed up it does fine going uphill at 70 it starts doing it again. Someone said to replace the ignition module is this correct. I put new plugs, wires, o2sensor egr valve cap and fuel filter on about 600 miles ago this has the efi system. Please help I am not that familiar with the 454 motor and the efi system. I have also heard that the dist. shaft cam wear out on these engines. would that cause this type of problem."
then the 2 responses:
"My 91 454 did the exact same thing. Had a miss at a certain MPH range.
Mine acted up between 57 and 65 mph, before or after this range it ran like a top.
The fix was a new distributor. As you've heard, the bearings wear out and the distibutor shaft wobbles. At specific rpm's it caused the miss which corresponded with the mph range.
Good as new, once replaced. "
and
"I'd second the distributor. Sounds like you've hit most of the other items. Understand the Throttle position sensor can also be a nominee after about 65-75,000 miles or so."
from http://www.campingworld.com/cforum/i...g/12901293.cfm

i will probably still do the EGR to be safe but the whole distrubutor will also be down the line if the EGR doesn't fix it...
also if the dist is the problem then how hard is it to replace? I am mechanically inclined but i don't know what to do, should i take it to a mechanic or try it myself?

ok replaced the EGR at work (needed a 1/2" wrench since all i have is sockets) and still have the missing/hesitation during the drive home.. also replaced the vacuum lines from EGR to Electronic thing and elec thing to front of throttle body.. i did notice that the electronic thing has a filter???
so next step is to do the whole distributor assembly... wonder how tough that will be

The distributor is not all that hard to do. You'll need a timing light, and a distributor wrench makes the job a lot easier. I'm pretty sure Autozones and the like rent timing lights if you don't have/can't borrow one. The Haynes manual actually covers the replacement of this pretty well, it may be worth it to pick one up.

Basically, you want to put the engine on top dead center on the #1 cylinder, on the compression stroke (piston at the top of the bore). If you take the spark plug out of the #1 cylinder, and have an assistant turn the crank with a wrench, you'll feel the compression when reaching TDC. Then, take off the cap and set it aside. Loosen the distributor clamp, and slide it out of the way. Note the general position of the distributor, take a picture if you need to. Pull the distributor up and out of the engine, and install the new one. Note that the oil pump is driven by the distributor shaft, you'll need to align that when installing. Also, the gears are helical cut, and the rotor will advance as you're installing that. You want to install it so that the rotor points at the #1 tower of the distributor cap when at TDC on #1.

Reinstall everything, snug the clamp, and check/set the timing.

Good luck!

Hey!

A buddy of mine just swapped out his old points distributor for an electronic HEI with the internal coil on his old International. It was a brand named DUI. Do any of you guys have experience or thoughts concerning making a swap like this or preference with a certain brand? I'd like to change out my external coil system for an enclosed newer aftermarket one, like the Mallory (I like the red cap).

Also, when you talk about changing the EGR, are you talking about just the valve, or the electronic assembly that it connects to? I have a similar problem with mine, it has a hard miss under acceleration. I was thinking it was a dirty fuel filter, but I'm getting an EGR light/code after driving for a couple of miles. What if you just block it off and disconnect the harness???

Thanks!

i just replaced the valve since that was the main area of concern when i did my searching. I also replaced my feul filter as well thinking thats what it was to start with. Now with new feul filter, new EGR valve, new spark plugs and wires... based on what i have seen and read it seems the distributor is the culprit...

Re: strange hesitation
 

after all this time, with work and daughter and everything I haven't had anytime to look into this...
Now it has expanded it speed range at which it happens. Originally it only happened from 50-55mph.. now it happens from 40-60mph... it accellerates fine maybe a slight pause then kicks in but as soon as you let off the gas to maintain speed or "cruise" is when it kicks in. Just feels like it is losing power but most of the time on level streets it does maintain speed but uphill it does take a few seconds to kick in properly...
I am trying to decide if I should just replace the upper portion of the distributor (cap, rotor, module and whatever else is in there) or if i should just spend the extra $60 for the entire setup (Distributor, A-1 Remanufactured, $98.99), especially since I don't have a whole lot to spend on parts and time is limited.
I may start with the above suggestions by 1987SierraClassic:
*EGR - could be carboned up and sticking.
*Injectors - could be dirty
*Vacuum leak
*Dist. Cap - could be cracked.
*Timing - Local garage timed mine without disconnecting the diag. lead...ended up being really sluggish until I showed them how to time it properly.
*Throttle Position Sensor - this may or may not set a code.
*Coil - maybe, though unlikely.



Basically I am looking for other suggestions and any updated information anyone may have for me.

Re: strange hesitation
 

a long time GM guy at work suggested it may be the Throttle Position Sensor which might also help explain the high idling at startup... this is something else I will look into

Re: strange hesitation
 

how hard is it to replace the TPS and is there any online how tos on replacing it? I can get it locally for $22 but not sure how hard it is to replace and what to look for with bad/worn parts. I don't really want to go spend $30 for a crappy Haynes manual but if I really have to I will...

Re: strange hesitation
 

kinda off the subject, but I got a Haynes manual from Amazon's used site for $2 and another $2 for shipping. Really couldn't beat the price. Good luck with your hesitation....
MAS

Re: strange hesitation
 

i'll look into that... thanks!

Re: strange hesitation
 

update: Sunday I took the time to do some checking and got a TPS ($22) and haynes manual ($16) and took me 5 minutes to replace it... It sits on the passenger side of TBI. Just needed to remove 2 screws, unhook the wiring harness, take it off, put new one on and tighten down. Also had to remove the ground for 20 seconds to reset the codes because the burb wouldn't start after replacing it for some odd reason.

PROBLEM SOLVED!

now for the other problems, will make a new post for those...

Re: strange hesitation
 

Glad to hear you got it solved, sorry I missed your posts about the recent events. I didn't have much of a chance to get on the computer lately.

So it wasn't throwing any codes for the TPS, but it was indeed bad? If so, this may be why my 87 stalls randomly too....

Re: strange hesitation
 

Hi to all, just found this site and you guy's are great.. My name say's it all. I just got a 91 suburban 4x4, love this thing. It's got the same (HESITATION) thing going on. Around 50-60 mph, getting ready to take it to a damm good mechanic. I have read your post and took some note's on what to look for, in suburban's. I am thinking it could help, so nothing is over looked. I also have a loud knocking every time I start it about 4 or 5 knock's and then it stop's, and run's find beside's the hesitation. Glad I found this site a will post some more pix's soon . My suburban has about 127,500 miles on it (350-5.7 liter)NEW GMC FAN.

Re: strange hesitation
 

The knock on startup is most likely low oil pressure at startup, or thick oil.

I've found that some of the cheaper oil filters don't have anti drainback valves, which causes the oil to drain back into the pan, and a "dry startup" along with the rod knock you described. I've been using AC Delco filters on it, and have not had that problem since. I also had to switch to 5w30 for winter use. 10w30 seemed too thick for the colder temps.

And you're right, this is the best group of guys that I've been associated with regarding these trucks. :metal:

Re: strange hesitation
 

Hi to all, Thanks for the info about the oil and filter's. Don't really know what's on there now but will try what you said. I have to change the oil soon away. Hope it work's and thank you again.

Re: strange hesitation
 

Glad to help, let us know how you make out!

Re: strange hesitation
 

Hi to all, how I make out is for my lady to know....But I will let you know what's up with the suburban.......LOL

Re: strange hesitation
 

:o

Guess I should be more careful in how I word things. :lol: