Im having the same problem with my truck between 60 and 70 mainly between 1800 and 2300rpm. I started a thread on it but it doesnt seem to be getting much attention, so I'm watching this thread pretty close hoping it will reveal something I can try.

 

Well, I was having the SAME problem. Intermittant on the interstate, steady throttle at 65mph, slight hesitation, maybe a knock, then the torque converter unlocks and lets the engine spin up to 2300rpm and everything is fine. Started doing it a few weeks ago, tripped the SES light (multiple random misfire), then when driving around town it would go out.

Plugs, wires, cap and rotor all replaced in the past 3 months. I had taken it in to a shop to check for a head/intake gasket leak a month ago when I noticed a slight loss of coolant ~ half gallon every 3000 miles. Engine never overheated, oil always looked good, never any external leak I could find. They said they found a loose hose clamp.

Well, TODAY, on my way home from work, the temp gauge shot up past 200F. When I pulled over and shut down, it was up to 240. Coolant was fine, but the oil was above the F, and milky. Steam came out of the filler neck as well.

Right now its in the shop, they'll call me tomorrow, but what I THINK has been happening was I had a leaky head gasket for the past few months, but it never leaked enough to contaminate the oil, and it finally blew today.

Now, could this be related to a low end miss? I'm not sure. But low compression on one or more cylinders, coolant leaking into the combustion chamber...

Hopefully I know tomorrow if THAT was the issue causing the miss...

Here's hoping that the Mobil1 I've been running protected the rest of my motor. I don't think I went more than a few minutes after it bit the big one...

(FWIW, '99 Jimmy 4WD)

 

It is more likely that the intake manifold gasket had partially failed and was allowing coolant into the cylinder causing an intermittent miss. After a time, it finally failed allowing coolant into the oil... The intake manifold gaskets are a lot more common of a failure than head gaskets.

I would insist that the shop you have looking at it do a compression check. If it's low on one or more cylinders, I would suspect a head gasket. If they are relatively equal pressures, then I wouldn't dig past the intake manifold gaskets.

 

I sorta agree here, however, I would recommend a leak down test, as opposed to just a compression test. If you have excessive leakdown on any of the cylinders, or, get bubbles in the radiator, NOW would be the time to fix it..... (provided it is a top end problem for the leak down, and not rings......)

 

I picked up the Blazer from the shop today, they said the SES showed it was missing (duh) but as it warmed up the miss became less frequent , I knew that, but they could find no reason for the miss, they say they checked all components of the electrical system (plugs , they are double AC Platnums, wires, cap, rotor) and still believe that the problem is the spyder assy. they believe it is the popet valves sticking, they are 99 % sure this will fix it. They want $600 for the job. They never mentioned the coil, hmmm, I still believe the issue is with the electrical system as it shows up the most during wet / damp weather. If I do go with the spyder change I believe it is something I can do ( I have restored a couple of AMC 'S ) and do have a decent understanding of mechanics. Any tips from here.
Thanks again to all........AMXAL

 

Well, I got the Blazer back from the shop after 2 days, they say that they could find nothing wrong, but admitted it does miss and believe it is the spider assembly (they are 90% sure) they want $560 to fix. Good thing about this is they did not charge for the diagnois, wish Doctors were that generous! Anyway, one of my friends suggested a new coil, I got the original one out and found 4 cracks and a couple of burn marks, it does run alot better, seems to have more acceleration, but I still have a miss between 60 and 70 mph. I have read all the input about a spider, I have done a lot of auto repair and would consider myself above average, what is involved in the spider replacement, drain coolant? remove alt. a/c, air plenum?? Any help is always appreciated, Thanks to all, AMXAL

 

No need to drain coolant. Remove the intake plenum. Follow the wiring harness that runs over the top of the engine and disconnect it from underneith of the fuse/relay center (you'll have to unbolt the fuse/relay center to do so), disconnect from all sensors etc and lay it on the passenger fender. Release fuel pressure, remove fuel lines from the top of the engine. From there, you've just got to remove the upper plenum bolts and the upper plenum. It may release kinda hard from the seal around the injection spider main body, but it will let go sooner or later. A bit of WD 40 where the injector body meets the upper plenum will help. From there you just slide the poppet nozzles out of their bores and unclip the spider from the mount.

I probably forgot a few steps, but you get the picture.

But again, I have to ask how annoying this stumble is, especially now after you have done some things that have had an impact. I had a VERY slight stumble/miss that I could feel, but no one else could and thought the same thing, spider... The spider helped, but it was still there.

 

Kyle,
Thanks for the info, you bring up a good point about the miss, and me being the only one that feels it, I tried to point it out to my wife the other night and she couldn't feel it, I think what I'm going to do is run it for a few days ( besides it suppose to get into the teens for temp the next few days and it will be to cold to work on it) check the milage and see how it goes, I do use it for weekly trips of 188 miles on the freeway so I do run 65 to 70+ . At least I know whats ahead if I go that way. Thanks again for your help. By the way I see your working on a 70 Chevelle, I'm restoring a 68 AMX.
AMXAL

 

Does anyone think that some fuel injection cleaner would help??? or hurt?

 

My experience was that it would help, but only temporarily. As soon as that tank was gone, it was back to a very slight stumble. If the problem is not very noticable and isn't impacting the performance or relative efficiency of the vehicle...

With my vehicles (especially the lower mileage ones), I run fuel injector cleaner and fuel system treatment at each oil change (different tanks of fuel though) just to keep deposits from forming. The treatment will keep sulfur deposits from fouling your fuel level sender and the cleaner will keep your injectors clean. On my Rainier, I spend $45 per oil change for a filter, oil (7 qts [sm=yikesomg.gif]), a large bottle of Chevron w/ Techron, and a bottle of GM Fuel System Treatment.

On my old Bravada, I purchased it with 72k miles and it was clear that it had not been maintained such as I would have maintained it. With regular use, fuel system treatment and/or fuel injector cleaner can keep things working as they are supposed to for a long time to come. The problem in most cases is the crappy fuel that we are paying more and more money for.